























































































































































V 








TECHNICAL LIBKAKx 


of the 


ARMED FORCES 
SPECIAL WEAPONS PR0JJ5 

3 - n • £L ... 






I 



SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 



UNCLASSIFIED 



aasafie;] 



Manuscript and illustrations for this volume were prepared 
for publication by the Summary Reports Group of the 
Columbia University Division of War Research under con¬ 
tract OEMsr-1131 with the Office of Scientific Research and 
Development. This volume was printed and bound by the 
Columbia University Press. 

Distribution of the Summary Technical Report of NDRC 
has been made by the War and Navy Departments. Inquiries 
concerning the availability and distribution of the Summary 
Technical Report volumes and microfilmed and other refer¬ 
ence material should be addressed to the War Department 
Library, Room 1A-522, The Pentagon, Washington 25, D. C., 
or to the Office of Naval Research, Navy Department, Atten¬ 
tion: Reports and Documents Section, Washington 25, D. C. 

Copy No. 

2.S2 


This volume, like the seventy others of the Summary Tech¬ 
nical Report of NDRC, has been written, edited, and printed 
under great pressure. Inevitably there are errors which have 
slipped past Division readers and proofreaders. There may 
be errors of fact not known at time of printing. The author 
has not been able to follow through his writing to the final 
page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports 
and sent to recipients of the volume. Your help will make 
this book more useful to other readers and will be of great 
value in preparing any revisions. 




SUMMARY TECHNICAL REPORT OF DIVISION 17, NDRC 

VOLUME 1 

DETECTION OF LAND MINES 
AND SOUND RANGING 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 17 

GEORGE R. HARRISON, CHIEF 


WASHINGTON, D. C., 1946 


iUPSH 

UNCLASSIFIED 





NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative 1 

Frank B. Jewett Navy Representative 2 

Karl T. Compton Commissioner of Patents 3 

Irvin Stewart, Executive Secretary 

2 Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 
Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 
Commodore H. A. Schade 

3 Commissioners of Patents in order of service: 

Conway P. Coe Casper W. Ooms 


1 Army representatives in order of service: 

Maj. Gen. G. V. Strong Col. L. A. Denson 

Maj. Gen. R. C. Moore Col. P. R. Faymonville 

Maj. Gen. C. C. Williams Brig. Gen. E. A. Regnier 

Brig. Gen. W. A. Wood, Jr. Col. M. M. Irvine 

Col. E. A. Routheau 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suit¬ 
able projects and research programs on the instrumen¬ 
talities of warfare, together with contract facilities for 
carrying out these projects and programs, and (2) to 
administer the technical and scientific work of the con¬ 
tracts. More specifically, NDRC functioned by initiating 
research projects on requests from the Army or the 
Navy, or on requests from an allied government trans¬ 
mitted through the Liaison Office of OSRD, or on its 
own considered initiative as a result of the experience 
of its members. Proposals prepared by the Division, 
Panel, or Committee for research contracts for perform¬ 
ance of the work involved in such projects were first re¬ 
viewed by NDRC, and if approved, recommended to the 
Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of 
the contract, including such matters as materials, clear¬ 
ances, vouchers, patents, priorities, legal matters, and 
administration of patent matters were handled by the 
Executive Secretary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 

These were: 

Division A—Armor and Ordnance 
Division B—Bombs, Fuels, Gases, & Chemical Problems 
Division C—Communication and Transportation 
Division D—Detection, Controls, and Instruments 
Division E—Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three 
administrative divisions, panels, or committees were 
created, each with a chief selected on the basis of his 
outstanding work in the particular field. The NDRC 
members then became a reviewing and advisory group 
to the Director of OSRD. The final organization was 
as follows: 


Division 1—Ballistic Research 

Division 2—Effects of Impact and Explosion 

Division 3—Rocket Ordnance 

Division 4—Ordnance Accessories 

Division 5—New Missiles 

Division 6—Sub-Surface Warfare 

Division 7—Fire Control 

Division 8—Explosives 

Division 9—Chemistry 

Division 10—Absorbents and Aerosols 

Division 11—Chemical Engineering 

Division 12—Transportation 

Division 13—Electrical Communication 

Division 14—Radar 

Division 15—Radio Coordination 

Division 16—Optics and Camouflage 

Division 17—Physics 

Division 18—War Metallurgy 

Division 19—Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


»v 


CONFIDENTIAL 



NDRC FOREWORD 


A S EVENTS of the years preceding 1940 re- 
L vealed more and more clearly the serious¬ 
ness of the world situation, many scientists in 
this country came to realize the need of organ¬ 
izing scientific research for service in a national 
emergency. Recommendations which they made 
to the White House were given careful and 
sympathetic attention, and as a result the Na¬ 
tional Defense Research Committee [NDRC] 
was formed by Executive Order of the Presi¬ 
dent in the summer of 1940. The members of 
NDRC, appointed by the President, were in¬ 
structed to supplement the work of the Army 
and the Navy in the development of the in¬ 
strumentalities of war. A year later, upon the 
establishment of the Office of Scientific Research 
and Development [OSRD], NDRC became one 
of its units. 

The Summary Technical Report of NDRC is 
a conscientious effort on the part of NDRC to 
summarize and evaluate its work and to present 
it in a useful and permanent form. It comprises 
some seventy volumes broken into groups cor¬ 
responding to the NDRC Divisions, Panels, and 
Committees. 

The Summary Technical Report of each Divi¬ 
sion, Panel, or Committee is an integral survey 
of the work of that group. The first volume of 
each group’s report contains a summary of the 
report, stating the problems presented and the 
philosophy of attacking them, and summarizing 
the results of the research, development, and 
training activities undertaken. Some volumes 
may be “state of the art” treatises covering 
subjects to which various research groups have 
contributed information. Others may contain 
descriptions of devices developed in the labora¬ 
tories. A master index of all these divisional, 
panel, and committee reports which together 
constitute the Summary Technical Report of 
NDRC is contained in a separate volume, which 
also includes the index of a microfilm record 
of pertinent technical laboratory reports and 
reference material. 

Some of the NDRC-sponsored researches 
which had been declassified by the end of 1945 
were of sufficient popular interest that it was 
found desirable to report them in the form of 
monographs, such as the series on radar by 
Division 14 and the monograph on sampling 
inspection by the Applied Mathematics Panel. 
Since the material treated in them is not dupli¬ 
cated in the Summary Technical Report of 
NDRC, the monographs are an important part 


of the story of these aspects of NDRC research. 

In contrast to the information on radar, 
which is of widespread interest and much of 
which is released to the public, the research on 
subsurface warfare is largely classified and is 
of general interest to a more restricted group. 
As a consequence, the report of Division 6 is 
found almost entirely in its Summary Technical 
Report, which runs to over twenty volumes. 
The extent of the work of a division cannot 
therefore be judged solely by the number of 
volumes devoted to it in the Summary Tech¬ 
nical Report of NDRC: account must be taken 
of the monographs and available reports pub¬ 
lished elsewhere. 

The research work of Division 17 included a 
wide variety of projects, ranging from the de¬ 
tection of land mines to the characteristics of 
the human ear, from helium purity indica¬ 
tors to the telemetering of strain gauges, from 
odographs to sound-ranging devices. It is a 
tribute to the broad knowledge of the Division 
Chiefs—Paul Klopsteg and, later, George R. 
Harrison—and to the versatility of the men 
who worked under them that so diverse a pro¬ 
gram was handled so competently. 

A considerable portion of the work of Divi¬ 
sion 17 had to do with the shattering noise of 
modern war, and answers were sought and sup¬ 
plied to such questions as: How much noise can 
a human being stand? What clues must the 
human ear have in order to understand a 
spoken message? How much distortion can be 
tolerated? These and other phases of the Divi¬ 
sion’s work are dealt with in the Summary 
Technical Report prepared under the direction 
of the Division Chief and authorized by him 
for publication. 

The diversity of the Division’s projects made 
it inevitable that its staff should be composed 
of men with many types of scientific training 
and that the Division should draw on contrac¬ 
tors with a wide range of experience and skills. 
The studies of noise, in particular, meant that 
the technical staff must include physicists, 
acousticians, and psychologists. For the ability 
and devotion of these men of many aptitudes 
we express our gratitude. 

Vannevar Bush, Director 
Office of Scientific Research and Development 

J. B. Conant, Chairman 

National Defense Research Committee 


CONFIDENTIAL 


v 



















FOREWORD 


T he applied physics division of the Office 
of Scientific Research and Development 
[OSRD] was organized late in 1942 under the 
chairmanship of Dr. Paul E. Klopsteg, who was 
responsible for the work of the Division until 
shortly before the completion of its work when 
other duties required his full attention. Most 
of the projects which had been initiated by the 
Instruments Section of the National Defense 
Research Committee [NDRC] during 1940 and 
1941 and which were not concerned with optics 
were turned over to the Applied Physics Divi¬ 
sion on its inauguration. Dr. Klopsteg as Chief 
and Dr. E. A. Eckhardt as Deputy Chief went 
with them from the Instruments Section to the 
new Division. 

The Summary Technical Report which is pre¬ 
sented in these volumes thus covers the accom¬ 
plishments of projects set up by both Section 
D-3 and Division 17. The work of the Division 
covered a very wide range of fields. The term 
Applied Physics served in lieu of a more de¬ 
scriptive name for a Division which was in fact 
the one to which was assigned any scientific 


problem which did not properly come under one 
of the other divisions of NDRC. 

Actually the Division was an association of 
three Sections having rather dissimilar respon¬ 
sibilities and fields of activity. In setting up 
these Sections it was necessary to group the 
projects already under way into a small num¬ 
ber of coherent categories, and those chosen 
were Sound, Electricity, and General Instru¬ 
mentation. The work of the Division consisted 
entirely of the integrated efforts of these three 
Sections, whose membership will be found listed 
on a succeeding page. 

For more detailed reports on the technical 
work of the Division than are contained here¬ 
with the detailed contractors’ reports of Divi¬ 
sion 17 should be consulted, and appropriate 
reference to these have been made throughout 
the present volumes. The results obtained are 
also presented in less technical form in that 
volume of the history of OSRD entitled Optics 
and Applied Physics in World War II. 

George R. Harrison 
Chief, Division 17 


vii 


CONFIDENTIAL 





PREFACE 


T he research and development program of 
Division 17, NDRC, was concerned with 
those problems in physics not specifically cov¬ 
ered in other divisions of NDRC. As the result, 
the Division fell heir to a myriad of miscella¬ 
neous problems of a physical nature which, in 
themselves, were not often interrelated. It would 
have been exceedingly difficult, if not im¬ 
possible, for Division 17 to set up a sufficient 
number of sections to deal specifically with all 
the various classes of problems which fell under 
its jurisdiction. Therefore, the projects of the 
Division were assigned to one of three sections 
—Section 17.1, Instruments; Section 17.2, Elec¬ 
trical Equipment; and Section 17.3, Acoustics— 
whose broad titles permitted a general, even if 
somewhat loose, classification. It was not always 
easy to decide, at times, under which of these 
three broad categories a given project should be 
placed. In such cases, considerations such as 
immediate convenience and availability of ex¬ 
perienced personnel were often the determining 
factors. 

The Summary Technical Report describing 
the activities of Division 17 is presented in four 
volumes. In an attempt to achieve a little greater 
uniformity of subject matter, the projects were 
organized within the various volumes without 
regard to their section classification. Conse¬ 
quently, there is, on the whole, little relation¬ 
ship between volume and section number. Be¬ 
cause of the varied problems dealt with in the 
Division’s program, very little continuity is to 
be found from chapter to chapter in any volume. 
Each chapter attempts to summarize independ¬ 
ently the results of a particular project. 

Since there were a large number of diversified 
projects in Division 17, it was obviously im¬ 
possible to do justice to each, even in summary. 
It is not intended that the importance of any 
project described herein should be judged by 
the amount of page space allotted to it. Nat¬ 
urally, certain problems involved more research 
and development than others before they could 
be brought to a successful conclusion. In many 
cases, this is reflected in the Summary Tech¬ 
nical Report. On the other hand, the presenta¬ 
tion of the projects may mirror the enthusiasm 
(or lack of it) of the individual author at the 
time of writing. Therefore, the reader who 
desires more than a broad panorama of the 
Division’s activities is referred to the Microfilm 
Index for more complete details. 


An attempt has been made in this first 
volume of the Division 17 Summary Technical 
Report to group together the projects dealing 
with the development of detecting devices for 
various purposes and methods of detection of 
certain objects. Not all the projects within the 
Division that conceivably could be classified 
under these two categories are included. In 
general, only those detecting devices or methods 
which were developed for specialized or unique 
purposes found their way into this volume. 

The first three chapters of this volume discuss 
a related group of projects while the remainder 
present the results of unrelated researches. An 
effort has been made to see that each chapter 
presents the problem clearly, outlines the meth¬ 
ods of attack, and states the important results 
or conclusions. Every reasonable effort has been 
made to keep this volume free from error, scien¬ 
tific or otherwise. Should some creep in, the 
authors and editor would be grateful to have 
them called to their attention. Although this is 
a technical report, by its very nature it is in¬ 
evitable that occasionally opinions other than 
scientific are expressed. These do not neces¬ 
sarily reflect the opinions of the authors, editor, 
or NDRC as a whole, but rather those of Di¬ 
vision 17. 

It should be borne in mind that the material 
contained in these chapters represents a sum¬ 
mary of the combined efforts of many men who 
labored so faithfully for so long for so little per¬ 
sonal recognition. Although certain of these 
men, as authors, bore the brunt of preparing 
this volume, many others contributed freely of 
their time to read the authors’ manuscripts to 
check for accuracy, Division and NDRC policy, 
to offer invaluable suggestions and criticisms, 
and to answer innumerable questions. To these 
men, then, as well as to the authors, the editor 
of this volume expresses his deep appreciation. 

Lest the mention by name of all those who 
contributed to the preparation of this volume be 
considered as a listing in a minor hall of fame, 
it has been purposely avoided. The name of the 
author will be found under the chapter titles. 
Where no name appears, the editor, with the 
help of innumerable Division members, Con¬ 
tractor employees, and friends, has prepared 
the summary from the various contractors’ re¬ 
ports. 

Chas. E. Waring 
Editor 


CONFIDENTIAL 


IX 








CONTENTS 


CHAPTER PAGE 

1. Detection of Land Mines. 1 

2. Mechanical and Demolition Clearance of Land 

Mines by John S. Hornheck .46 

3. Magnetic Characteristics of Vehicles and Magnetic 

Land Mines.67 

4. Control System and Detectors for Submarine In¬ 
fluence Mines by F. L. Yost .79 

5. Gun Ranging and Locating Systems by Frank 

Woodbridge Constant .96 

6. Infrared Gas Detectors and Analyzers by Clark 

Goodman .176 

7. Detection of Plastic Particles.182 

8. Locating Unexploded Bombs by F. L. Yost . . 187 

9. Development of an Electromagnetic Mass Detecting 

Security Device by F. L. Yost .193 

10. Surgeon’s Metal Locator.201 

Glossary.203 

Bibliography.205 

OSRD Appointees.211 

Contract Numbers.213 

Service Project Numbers.219 

Index.221 

CONFIDENTIAL xi 


















Chapter 1 

DETECTION OF LAND MINES 


41 INTRODUCTION 

111 Purpose of Report 

T his report is designed to summarize for a 
technically trained reader the scientific pro¬ 
gram conducted by Section 17.1 and its pred¬ 
ecessor, Section D-3, of the National Defense 
Research Committee [NDRC], for the develop¬ 
ment of land mine detectors. It is to be sub¬ 
mitted, along with the other summary technical 
reports of this series, to the Army and Navy in 
fulfillment of final obligations of NDRC. 

1,1,2 Organization and Scope 

It is intended that this report will present 
sufficient background to enable the reader to 
understand why and how the various problems 
were undertaken, and it is intended to present 
a clear picture of the “state of the art” at the 
termination of the work. Emphasis is given to 
the two detectors, one to locate metals and 
the other nonmetals, which were placed in large- 
scale production, as well as to the final experi¬ 
mental models nearing production which repre¬ 
sented the highest development state at the 
close of World War II. No attempt is made 
to include material sufficient to enable other 
investigators in this subject to continue the 
experimental program without resort to biblio¬ 
graphical reference material. An attempt has 
been made, however, to record the develop¬ 
ments in such detail that the next investigators 
will understand why certain attacks on the 
problems involved were rejected. 

The report is organized in five major sections. 
The introduction is intended primarily to orient 
the reader and to present a brief overall picture 
of the development program: its results, its 
evaluation, and the problems it left unsolved. 
The next three sections each deal with a gen¬ 
eral class of mine detectors. The breakdown 
of all detectors into these three classes is ad¬ 
mittedly arbitrary and open to reasonable 
objections; for example, many detectors do not 


fit uniquely into a single classification. It was 
under the classifications adopted for this report, 
however, that the work was organized through¬ 
out the course of the program. The fifth sec¬ 
tion deals with the problems which were cur¬ 
rent at the close of World War II, such as 
possible countermeasures and anti-countermeas¬ 
ures. Here, too, the implications of the trend 
in mine warfare are considered in terms of a 
future peacetime research program. 

1.1.3 Army Interest and Liaison 

In the Army Service Forces [ASF] the re¬ 
sponsibility for the development of mine de¬ 
tection equipment was delegated to the Chief 
of Engineers, and the responsibility for equip¬ 
ment production to the Chief Signal Officer. In 
the chain of command, the engineering respon¬ 
sibility fell to the Minefields and Fortifications 
Branch, Research and Development Division, 
Office, Chief of Engineers, which delegated the 
primary development responsibility to the 
Engineer Board, Fort Belvoir, Virginia, and 
its Applied Electronics Branch. Intimate liaison 
was maintained with these and other officers 
by Section 17.1, and earlier by Section D-3, 
throughout the course of the development. 

1.1.4 Service Projects 

The Service control numbers under which 
the NDRC program functioned were CE-4 and 
CE-31. CE-4 was accepted by NDRC on Febru¬ 
ary 21, 1941, and was terminated on October 
14, 1942. CE-31 was accepted by NDRC in 
January 1943 and was a continuing project 
until the end of hostilities. 

I 2 SUMMARY 

Heavily mechanized and mobile warfare has 
been generally recognized as an important 
characteristic of World War II. It was, there¬ 
fore, natural that certain defensive weapons 
came to the forefront as a means of limiting 


CONFIDENTIAL 


1 


2 


DETECTION OF LAND MINES 


the power and restricting the mobility of the 
offense. One of the most important of these 
weapons was the land mine. The strategy and 
tactics involved in the employment of land 
mines, including their use as a weapon of 
offense, was highly developed by the German 
army and, to a lesser extent, by the Italian 
and the Japanese. Before the United States’ 
entry into the war, the land mine had already 
emerged as a successful defensive weapon if 
properly employed, primarily through its use 
by the Afrika Korps in Libya and Cyrenaica. 
Thus, one of the earliest problems presented 
by the Army to NDRC was the development 
of a portable detector of land mines suitable 
for operation by foot soldiers. 

The development of equipment for the pas¬ 
sage of enemy minefields can be arbitrarily 
broken down into three groups: mine detection, 
mechanical clearance, and demolition clearance. 
Of these, mine detection is the most important 
as it was used much more extensively than 
any other type of equipment in combat war¬ 
fare for the penetration of enemy minefields. 
While mine detection and removal were origi¬ 
nally designated a function of combat engineer 
troops, the prolific use of anti-tank and anti¬ 
personnel mines by the enemy made it impos¬ 
sible for this assignment to be carried out in 
practice. The technique of mine detection and 
removal thus became, of necessity, familiar 
to most ranks and organizations within the 
Army. 


121 Military Requirements 

Military requirements for mine detectors 
changed during the war as the enemy intro¬ 
duced new mines and new techniques in plant¬ 
ing, and also as the battleground progressed 
from Africa to Italy, to the European theater 
of operations [ETO], and similarly in the 
Pacific. The initial Service request, designated 
Service project or directive CE-4, called for the 
development by Section D-3, NDRC, of two 
metal detectors—one capable of detecting non- 
ferrous metals and the other capable of detect¬ 
ing mines containing ferromagnetic parts. 
Specifically, they were required to detect a 
V 2 -lb metallic object at a depth of 24 in. With 


the development of detector set SCR-625 
through the combined efforts of NDRC and 
the Engineer Board, primary attention was 
directed by NDRC to fulfill the requirements of 
CE-31, calling for the development of a detector 
capable of locating explosives as such buried 
in the ground and nonmetallic anti-tank mines. 
With the introduction of the Schu mine a and 
other anti-personnel mines containing very 
little metal, military requirements changed to 
include the location of small nonmetallic mines 
and of extremely small amounts of metal, such 
as are used in fuzes and igniters, at consider¬ 
ably reduced depths of burial. The encountering 
of lava soil of high magnetic susceptibility in 
Italy and of other soils with magnetic charac¬ 
teristics in northern Europe and the Pacific 
theaters required that our mine detectors 
operate over such soils without producing 
spurious signals which would mask the mine 
signals. It was also recognized early that mine 
detectors must be able to operate over ground 
heavily littered with shrapnel and other battle 
refuse. 

Practically, military requirements for mine 
detectors were expressed in terms of the de¬ 
tection of known, widely used enemy mines at 
operational depths of burial under a wide 
variety of soil conditions. Anti-personnel mines, 
such as the Schu mine, the Mustard Pot mine, 
and the S mine, were rarely encountered at 
depths greater than 2 in. Most were found on 
top of the surface (camouflaged) or with less 
than t /2 in. of earth covering them. Anti-tank 
mines (such as the German Tellermine series, 
Riegel mines, and Topf mines) were usually 
buried not more than 6 or 8 in. below the 
ground, although there were many instances of 
anti-tank mines being buried to depths as great 
as 3 ft. In the Japanese theaters such enemy 
anti-tank mines as the Yardstick mine, the 
Type 3 Flowerpot, and the J-93 were rarely 
buried even 6 in. deep. The Japanese anti-per¬ 
sonnel mines were usually above, flush with, 
or just below the surface. 

The above requirements had to be fulfilled 
under battle conditions, involving detection at 
night, frequently under direct enemy fire. 47 

a For information concerning this and other mines 
that will be mentioned, see STR, Division 2, Volume 1, 
or Engineer intelligence bulletins. 


CONFIDENTIAL 




SUMMARY 


3 


An additional important consideration is that 
mine locating with a detector must be faster, 
safer, and at least as reliable as mine locating 
by probing methods. Probing, the locating of 
mines by prodding the ground (usually in a 
direction about 45 degrees with the vertical), 
is a tedious procedure—slow, yet quite reliable 
and safe if done by an expert. The method was 
widely employed throughout the war. Its effec¬ 
tiveness was the major reason for discarding 
a number of mine detector developments. 

1,2,2 Detector Developments 

Two detectors, developed with the coopera¬ 
tion of NDRC, were placed in large-scale pro¬ 
duction and issued to combat troops. The first 
of these was a metal locator designated by the 
Signal Corps the SCR-625, and the second, a 
so-called nonmetal detector, was designated 
detector set AN/PRS-1. At the close of World 
War II the development of a universal detector 
(that is, one combining the desirable features 
of the SCR-625 and AN/PRS-1) was nearing 
the production stage. The Signal Corps’ identi¬ 
fication for this instrument was the AN/PRS-6. 

Detector Set SCR-625 

The SCR-625, developed under an NDRC 
contract with the Hazeltine Service Corpora¬ 
tion, Little Neck, L. I., comprised, essentially, 
a modified Hughes bridge operating at a fre¬ 
quency of 1,000 c. The bridge coil assembly 
consisted of three concentric, coplanar coils of 
different diameters arranged so that the mutual 
inductance between the inner and outer coils 
and the middle coil was very nearly zero. The 
outer and inner coils were connected to an audio 
oscillator, and the middle coil was connected 
to an amplifier, the output of which was fed 
into a meter and a parallel resonator. When 
the bridge was balanced the coefficient of 
coupling between the oscillator coils and the 
detector coil was zero, and no signal was heard 
in the earphones. The introduction of metal 
into the a-c field produced by the oscillator 
coils disturbed the field and thus altered the 
zero coupling condition. The resulting signal 
was amplified and detected, indicating the 
presence of a mine in the field of the detector. 
Altogether 115,000 SCR-625 detectors were 


procured by ASF. The total cost of this pro¬ 
curement program was very close to $25,000,- 
000. The experimental development cost was 
about $150,000. 

Detector Set AN/PRS-1 

The AN/PRS-1, an original development by 
the RCA-Victor Division, Indianapolis, Indiana, 
under a contract with NDRC, is generally listed 
as a nonmetal detector even though it will 
frequently detect metallic mines. Its perform¬ 
ance in locating metals, however, is inferior 
to that of a detector like the SCR-625, so that 
it has been customary to refer to this model 
as a nonmetal detector. In this locator energy 
from a 300-mc oscillator is radiated into the 
ground by means of a simple dipole antenna 
and reflector system. The operation of the de¬ 
tector is dependent upon the fact that the 
antenna loading is affected by the presence of 
surrounding objects, both metallic and non- 
metallic. This may be considered as due to a 
reflection of radiation back to the antenna, or 
as a variation in the impedance presented to 
the antenna. The impedance change, in turn, 
is reflected back into the oscillator, causing a 
change in loss in the tank circuit of the oscilla¬ 
tor. The variation in grid current of the oscilla¬ 
tor tube is proportional to the loss in the cir¬ 
cuit and can be used as an indication of the 
change in load. A voltage developed in the grid 
circuit is used to control an oscillator-amplifier, 
and variations of the grid current produce aural 
indications in a resonator. At a cost of about 
$12,000,000 the Signal Corps procured 29,600 
AN/PRS-1 detector sets. Development costs of 
the AN/PRS-1 within NDRC totaled less than 
$ 100 , 000 . 

Detector Set AN/PRS-6 

The most promising NDRC detector still 
under development at the close of World War II 
was the AN/PRS-6, a universal mine detector, 
also the product of RCA-Victor, Indianapolis. 
This detector combines a modification of the 
PRS-1 with a metal detector of the mutual in¬ 
ductance bridge type; it represents merely an 
engineering advancement in the “state of the 
art.” The nonmetal-detecting portion of this 
instrument is designed to supplement the metal¬ 
detecting portion. It was expected that the 


CONFIDENTIAL 



4 


DETECTION OF LAND MINES 


metal-detecting system would, in practice, be 
sensitive not only to the large anti-tank mines, 
but also to mines containing extremely small 
amounts of metal. The nonmetal-detecting por¬ 
tion is sensitive primarily to anti-tank non- 
metallic mines, such as the Topf mine and 
certain variations of the Japanese Type 3 
Flowerpot mine. The major weakness of this 
detector is its inability to detect small anti¬ 
personnel nonmetallic mines. Increasing the 
sensitivity of the nonmetal-detecting portion 
would increase the possibilities of locating this 
type of mine, but at the same time would more 
than linearly increase the number of false in¬ 
dications due to roots, stones, and similar ob¬ 
jects. It is believed that a large number of false 
indications constitute a more serious objection 
than the lack of sensitivity to anti-personnel 
nonmetallic mines. 

Other detectors developed by NDRC which 
deserve mention are: (1) the portable iron de¬ 
tector [PID], (2) the beach detector, (3) seis¬ 
mic detectors, (4) the earth current detector 
[ECD], and (5) radioactive detectors. Brief 
descriptions of these instruments follow. 

Portable Iron Detector and the AN/PSS-1 

The portable iron detector was developed 
by the Carnegie Institution of Washington, 
D.C., the NDRC contractor, under project 
OD-46. The original development called ex¬ 
pressly for a device capable of detecting land 
mines in ferrous cases and suitable for attach¬ 
ment to a vehicle. The PID is, in effect, an ex¬ 
tremely simple magnetic gradiometer or, more 
exactly, a space-time gradiometer. It consists 
of two magnetometers aligned with each other, 
connected with their outputs in opposition. 
When swept over a ferromagnetic object, one 
magnetometer, being nearer to the object, will 
receive a stronger signal than the more remote 
magnetometer, while the uniform magnetic field 
of the earth affects both magnetometers equally 
(if the circuit is properly designed and 
balanced). Thus detection of ferromagnetic ob¬ 
jects is achieved without aligning the measur¬ 
ing devices in any particular direction with 
reference to the earth's magnetic field. Each 
magnetometer comprises a large number of 
turns of small (No. 40) insulated copper wire 


wound on a Permalloy rod. The detected signal 
is amplified and fed into a resonator. 

Although this device is light and sensitive, its 
inability to detect the nonferrous mines used by 
the enemy is a serious shortcoming. For this 
reason, the detector was not placed in produc¬ 
tion. It proved important, however, in furnish¬ 
ing a basis for the development and subsequent 
small-scale production of the AN/PSS-1, a de¬ 
tector designed for use by swimming members 
of naval combat demolition teams for the de¬ 
tection of anti-boat mines. The modification of 
the PID to the AN/PSS-1 was carried out under 
an Engineer Board contract. The principal 
modification was the addition of a battery- 
operated motor, which mechanically rotated the 
magnetometer coils, making it unnecessary for 
the operator to sweep the detector continuously 
in order to pick up a mine signal. A second 
change was the elimination of the Permalloy 
rods from the gradiometer; this made it pos¬ 
sible to increase the stability of balance and, 
therefore, the sensitivity. As all known enemy 
anti-boat mines contained some ferrous metals, 
the insensitivity of this device to nonferrous 
metals was not considered a serious objection. 

Beach Detector or AN/PRS-5 (XB-2) 

The beach detector, a development under 
NDRC contract by Electro-Mechanical Re¬ 
search Company, Houston, Texas, is a variation 
of the mutual inductance bridge-type detector 
which contains certain novel circuit design fea¬ 
tures. It is operated at a frequency of 465 kc; 
the use of this frequency enhances the conduc¬ 
tion effect as a phenomenon responsible for 
detection. This accounts for one unusual prop¬ 
erty: practically 100 per cent detection is ob¬ 
tained for both metallic and nonmetallic mines 
buried in soils of high conductivity, such as the 
region on a beach below the high-tide line. Over 
less-conducting soil the detector’s performance 
approaches very nearly that of the SCR-625 
and other standard metallic mine detectors. 

Seismic or Electromechanical Type 
Detectors 

Several attempts to use seismic methods to 
locate nonmetallic mines were made, including 
the following: (1) measurement of a variation 


CONFIDENTIAL 



SUMMARY 


5 


in the transmission of sonic vibrations between 
a vibrator and a pickup, each of which is in 
contact with the ground; (2) reflection of vi¬ 
brations from a mine due to the discontinuity 
it presents in the ground to a wave train or 
pulse propagated in the earth; (3) variation in 
the resonance properties of the earth over a 
buried object to acoustic vibrations, as com¬ 
pared to the adjacent earth (i.e., measurement 
of a variation in mechanical impedance). 

Instruments based on the third method 
proved that detection of both metallic and non- 
metallic mines is possible. No acoustic detector 
was placed in large-scale production, however, 
because field trials showed two disadvantages: 
(1) it is dangerous to use because contact must 
be made with the ground in areas where sensi¬ 
tive anti-personnel mines might be located; (2) 
the method proved to be slower and less sure 
than probing. The horizontal cross-sectional 
area of sensitivity is only about 1 sq ft, and 
the weight of the detector head is 6 lb or more. 

NDRC contractors actively engaged on these 
developments were the Sun Oil Company Geo¬ 
physical Laboratories, Beaumont, Texas, and 
RCA-Victor Division, Indianapolis, Indiana. 

Earth Current Detector 

A promising detector still under development 
at the close of World War II was the earth cur¬ 
rent detector, which was based on the British 
detector X-7. In this locator a ground loop of 
wire is laid out and energized by alternating 
current of frequency 1,000 c and of magnitude 
from 2 to 50 amp. A detecting-coil system car¬ 
ried by an operator is swept over the ground 
adjacent to and outside the ground loop. The 
detector coil is sensitive to anomalies in the 
a-c field induced by the current loop in the 
ground. A large area can be swept by the de¬ 
tector for one placement of the ground loop. 
This, to a certain extent, mitigates the follow¬ 
ing objections to the device: (1) a large amount 
of equipment is necessary (trailer-truck with 
auxiliary power supply) and (2) lack of flex¬ 
ibility in use. Initial experiments with this 
apparatus showed that it might be sensitive to 
both metallic and nonmetallic mines. Further 
experimental work seemed to prove that, for 
setups practical in the field, nonmetallic mines 


could not be detected using audio-frequency 
excitation. Detection of metallic mines ap¬ 
peared to be feasible at rather large depths of 
burial, particularly in the case of ferromagnetic 
objects. A special attribute of this detector 
arises from the fact that the detector itself has 
no field associated with it; this permits the 
detector to locate safely booby-trap mines fuzed 
with an igniter sensitive to the local field sur¬ 
rounding a detector of the SCR-625 type. The 
loop-energizing system of the ECD may also be 
useful for exploding harmlessly mines so fuzed. 

Development work on the ECD was carried 
out for Section 17.1 by the Shell Oil Company 
Geophysical Laboratories, Houston, Texas. 

Radioactivity Methods and “Mamie” 

Three other methods investigated for the lo¬ 
cation of nonmetallic mines were based on 
radioactivity measurements. In one it was at¬ 
tempted to measure the reduction of the natural 
radioactivity of the earth (a shielding effect) 
due to the presence of a buried object which is 
not itself radioactive. The procedure in this 
case was to sweep the ground with a sensitive 
Geiger-Mueller counter. A second method was 
based on detecting a variation in neutron and 
y-ray emissions from the ground when it is 
bombarded by a neutron source, such as a mix¬ 
ture of a radium salt with beryllium. The third 
method employed the procedure of measuring 
y-ray scattering when the soil is bombarded by 
y-rays from a portable source. Detection by 
these methods, in the main, was demonstrated 
to be sound in principle but impractical for 
military use. These investigations were con¬ 
ducted principally by the Texas Company Geo¬ 
physical Laboratory, Houston, Texas, and the 
Physics Department, Massachusetts Institute of 
Technology, Cambridge, Mass. 

“Mamie” is the code name for a method of 
marking friendly mines (usually nonmetal¬ 
lic). Small “buttons” containing radioactive 
cobalt-60 chloride, a y-ray emitter, are planted 
on top of or adjacent to a nonmetallic mine as 
a part of the standard operating procedure. 
The marked mines may then be relocated easily 
by using a sensitive y-ray detector of the 
Geiger-Mueller counter type. The Mamie 
scheme was developed independently by the 


CONFIDENTIAL 



6 


DETECTION OF LAND MINES 


Germans and in this country. Before the close 
of World War II in Europe it was intended to 
employ American y-ray detectors for the loca¬ 
tion of marked German Topf mines. The 
Physics Department, MIT, was responsible for 
this research under NDRC contract. 


Countermeasures and 
Anti-Countermeasures 

Electric influence fuzes for land mines, actu¬ 
ated by metallic detectors, have been developed 
both in this country and in Germany. Such a 
fuze was actually employed by the Germans 
against Russian mine detectors. It consisted of 
a few turns of wire, in the form of a coil, and 
a sensitive relay. Current induced in the coil 
by the magnetic field of a metallic detector of 
the SCR-625 type operated the relay, which in 
turn actuated an electric detonator. This device 
has been duplicated by the Signal Corps, and 
another influence fuze, of the proximity fuze 
type, was developed earlier by Section T, 
NDRC. A contractor of the Engineer Board has 
designed a metallic detector circuit which nulli¬ 
fies the effectiveness of the German-type fuze. 
This is accomplished by reducing the field 
strength put out by the field coil as the detector 
approaches a mine (i.e., the mine signal auto¬ 
matically reduces the current flowing in the 
field coil). The advantages of the ECD for com¬ 
bating this countermeasure have already been 
mentioned. 

Other types of countermeasures to mine de¬ 
tectors have been developed (e.g., the tilt ig¬ 
niter), and there is every reason to believe that 
more will be produced in the future. The entire 
field, including the design of both countermeas¬ 
ures and anti-countermeasures, has not yet been 
completely surveyed. 


Evaluation 

The SCR-625 detector set proved to be a suc¬ 
cessful and useful instrument in combat. It was 
employed by the U. S. Army throughout World 
War II, though it was obsolescent in the later 
stages. For example, it was necessary to make 
expedient field modifications in certain in¬ 


stances, enabling the detector to operate over 
soils of appreciable magnetic permeability. The 
ending of hostilities precluded the replacement 
of the SCR-625 by a better detector of the same 
type, known as the AN/PRS-3. It is now be¬ 
lieved that as a result of the early NDRC work 
on metallic mine detectors and, perhaps more 
important, later Engineer Board contracts, the 
problem of metallic mine detection has been 
solved rather completely, and the techniques 
involved in the design have been worked out 
satisfactorily. This statement applies particu¬ 
larly to locators of the mutual inductance type 
utilizing circuits which discriminate against re¬ 
active signals caused by magnetic permeability 
of the ground. Designs have also been com¬ 
pleted, under Army contracts, which include 
limited anti-countermeasure features without 
serious loss in sensitivity. 

Engineering design of special-purpose detec¬ 
tors (e.g., the beach detector and the PID) have 
not been brought to the same state of advance¬ 
ment. This can be accounted for by a lack of 
interest on the part of the using Services. 
While there is much to be said for the Service 
point of view, it can be argued logically and 
with considerable weight that modern warfare 
is most complex, requiring, in many instances, 
special-purpose equipment to perform certain 
tasks efficiently and safely. 

The entire field of countermeasures and anti¬ 
countermeasures involving metallic mine loca¬ 
tors is relatively new and unexplored. Vigorous 
investigations on this subject resulting in new 
techniques may change present concepts of me¬ 
tallic mine detector design radically. This field 
should not be neglected in formulating a peace¬ 
time research and development program. 

In evaluating the metallic mine detection pro¬ 
gram during and prior to World War II, one 
serious deficiency may be mentioned. While the 
SCR-625 was in its initial production stage 
(spring 1942), the phase discrimination princi¬ 
ple for improving its performance was under¬ 
stood and reduced to practice. But the impor¬ 
tance of this principle was not realized until 
some time later; by then it had been rediscovered 
independently by a contractor of the Engineer 
Board. The time loss could not be retrieved, 
with the consequence that no replacement for 
the SCR-625 was issued to combat troops. 


CONFIDENTIAL 



METALLIC MINE DETECTION 


7 


In the two and a half years after the com¬ 
pletion of the first prototype model of the 
AN/PRS-1, no major developments were made 
which can be said to have greatly improved the 
performance of nonmetallic and universal mine 
detectors. Engineering changes, however, were 
quite marked: the weight of the equipment was 
materially reduced, sensitivity was increased 
slightly, and spurious indications due to inade¬ 
quate height compensation were largely re¬ 
moved. The U-H-F detectors last developed 
were still unable to detect reliably small non¬ 
metallic mines with a major dimension less than 
four inches, and in certain soil conditions they 
could not reliably detect large anti-tank mines. 
Although these soil conditions occur only occa¬ 
sionally, they are still sufficiently frequent to 
destroy confidence in the detectors. The detec¬ 
tors are also plagued by false indications due to 
roots, stones, clumps of grass, weeds, and other 
conditions which are all too characteristic of 
practically any field on which a detector would 
be operated. It may be said fairly that not only 
is the problem of detection of nonmetallic mines 
inadequately solved, but also that no satisfac¬ 
tory solution had been obtained at the close of 
World War II. It seems clear that the problem 
will exist as long as armies fight on land or 
occupy enemy-held territory. It is concluded, 
therefore, that one of the important responsi¬ 
bilities of a peacetime research program is to 
carry on research and development toward the 
discovery of an adequate nonmetallic mine de¬ 
tector. The primary need is for a novel method 
of approach to the problem, since investigations 
based on methods described in this report have 
reached the point of seriously diminishing 
returns. Stated another way, this problem re¬ 
quires the basic research approach rather than 
engineering modifications of principles and 
methods employed heretofore. 


13 METALLIC MINE DETECTION 

The problem of the development of a metallic 
mine detector was first presented informally to 
NDRC by the Corps of Engineers in the fall of 
1940, and subsequently as project CE-4. At the 
time most of the nations at war had already 
developed and were using portable mine locator 


equipment. It was unfortunate that little or no 
information concerning these instruments, even 
about those developed by Britain and France, 
was available in this country. Thus, many 
widely varying approaches to the problem were 
considered. 


1,3,1 Possible Detection Principles 

Listed below are a number of the possibili¬ 
ties which early were discussed by the Hazel- 
tine Service Corporation, the original NDRC 
contractor, as a basis for the design of a metal¬ 
lic mine locator. 

Distortion of Earth’s Field 

Earth Inductor Gradiometer. Detection would 
be effected by detecting the distortion in the 
earth’s magnetic field in the vicinity of a mag¬ 
netic body, utilizing two astatic rotating coils. 

Variation of A-C Permeability by Distortion 
of Earth's Field. Distortion of the earth’s mag¬ 
netic field would be detected by astatic coils on 
cores of Permalloy, Perminvar, or other special 
magnetic materials, magnetized differently by 
the net field present at the individual coils. 

Variation of Self-Inductance 
Of a Single Coil 

This method would be applicable to any me¬ 
tallic body, not just a ferromagnetic one. The 
sensitivity would vary approximately with the 
inverse sixth power of the distance from the 
object. 

Variation of Balance of Inductance Bridge. 
When one of two coils of an inductance bridge is 
nearer the object than the other, the balance is 
disturbed. This method could be used at audio 
or radio frequencies. 

Beat-Frequency Method. Detection would be 
based on the variations of a frequency of oscil¬ 
lation, as controlled by the self-inductance of a 
single coil, by means of a beat note between the 
frequency of the search-coil circuit and a fixed 
frequency. 

Variation of Mutual Inductance 

Devices in this class employ separate units 
for transmitter and receiver. Both audio and 
radio frequencies are applicable. 


CONFIDENTIAL 



8 


DETECTION OF LAND MINES 


Audio-Frequency Device. A coil arrangement 
would be made such that zero coupling results 
between the sending and receiving coils in 
homogeneous space. The coupling change is 
detected and amplified directly. 

Radio-Frequency Device. This would be sim¬ 
ilar in principle to the audio-frequency device 
with the additional possibility of using a modu¬ 
lated carrier. 

Beat-Frequency Method. It was proposed to 
control the frequency of a beat oscillator by the 
mutual inductance between the sending and 
receiving coils. 

Reflection of Short Waves 

All the methods outlined above depend on the 
disturbance of a magnetic field in the vicinity of 
a metallic body. The distortion of the earth’s 
field is applicable only to magnetic bodies. The 
field associated with sending and receiving coils 
is entirely an induction field since the distance 
involved is only a small fraction of a wave¬ 
length. At frequencies so high that the radia¬ 
tion field becomes important at close range, 
there is a possibility of detection by means of 
the reflection of short waves. 


Military Characteristics 

Some of the early military characteristics and 
specifications for a metallic mine detector may 
be listed as follows. 

1. When a concentrated mass of not more 
than Vk lb of either steel, brass, or bronze, in 
the form of a cylinder, is buried in the earth 
at a depth of 2 ft, and the detecting element is 
moved in a horizontal plane 1 ft above the 
ground surface, the device shall detect and lo¬ 
cate the mass accurately. 

2. The location of the mass is to be indicated 
unmistakably both by a movement of a visual 
indicator and by an increase in the amplitude of 
the tone from a sonic resonator. The signal shall 
be a maximum when the locator is held ver¬ 
tically over the object. 

3. The device shall be capable of the above 
performance in all types of soil, whether wet or 
dry, and also when the metallic object is sub¬ 
merged in 2 ft of either fresh or salt water. 

4. The operation of the device shall not be 


affected by a soldier’s normal equipment, e.g., 
steel helmet, rifle, or bayonet. 

5. The device shall not interfere with radio 
reception, and its operation shall not be affected 
by radio transmission. 

6. The entire equipment shall be compact and 
capable of withstanding rough usage. 

7. The device shall be of suitable weight and 
construction for transportation and operation 
by one man. 

8. Equipment shall be designed to operate 
satisfactorily when the height of the detecting 
element above the ground surface is varied 
within a range of 4 to 8 in. When the detecting 
element is moved throughout this range, 
neither indicator shall give a signal comparable 
to that caused by a buried mine. 

9. The batteries shall have an operational life 
of not less than 8 hours. 

10. The device shall not respond to a large 
mass of metal, such as a vehicle, which is lo¬ 
cated with any part thereof placed at a hori¬ 
zontal distance of 4 ft from the center of the 
detecting element in its operating position. 

As World War II progressed, specifications 
for metallic mine detectors were modified and 
made detailed, as indicated by the following. 

1. Satisfactory detection shall be construed 
as an increase in rms voltage across the primary 
of headset HS-30 of 10 db or 40 mv, whichever 
is greater. 

2. At balance, the rms signal across the pri¬ 
mary of HS-30 shall be no greater than 40 mv. 

3. An 8-in. brass disk, % 6 in. thick, shall be 
satisfactorily detected 18 in. from the detector 
head. 

4. The detector shall locate satisfactorily the 
presence of a Type 3 Japanese fuze, less pres¬ 
sure spring, 3 in. from the search coil when the 
fuze body is buried flush in soil of volume 
susceptibility 100 times that of commercial 
FeCL-4 HoO. 

1,3,8 Development of the SCR-625 

By August 1941 the Hazeltine Service Cor¬ 
poration had completed the development of a 
mine locator, 13 ’ 20 based on an amplified fre¬ 
quency variation at 100 kc, which fulfilled or 
exceeded (with a few minor exceptions), the 


CONFIDENTIAL 



METALLIC MINE DETECTION 


9 


original requirements for a mine detector over 
average ground. The indicating note increased 
in pitch and volume as a metallic object was 
approached, but an exact null directly over the 
object resulted because of the coil configuration 
chosen. At that time it was tested compara¬ 
tively against a metal locator which had been 
previously developed by the Hedden Metal Lo¬ 
cators, Inc., of Miami, Florida, for other pur¬ 
poses. The Hedden locator was called to the 
attention of the Engineer Board by the Na¬ 
tional Inventors Council. It, essentially, was an 
audio-frequency mutual inductance bridge oper¬ 
ating at 1,000 c, with a two-coil transmitter- 
receiver system so arranged that there was 
normally no “mutual” between the transmitter 
and receiver. Comparative tests of the two lo¬ 
cators indicated that the audio-frequency type 
was more promising, since its circuits were 
simpler than the r-f locator. One of the prin¬ 
cipal objections to the Hedden locator was its 
extreme height sensitivity. Its detecting sensi¬ 
tivity, too, was probably inferior to that of the 
r-f detector. 

After this demonstration it was decided that 
Hazeltine should continue the development, 
using the principle of the Hedden locator. b 

Description of Instrument 


type output meter and to the audio resonator. 

Search Coils. The search coils, shown sche¬ 
matically in Figure 2, consist of three coplanar 
concentric coils. The radii and numbers of turns 
of the coils are so chosen that the mutual in¬ 
ductance between the outermost coil and the 
intermediate coil is numerically equal to that 
between the innermost coil and the intermedi¬ 
ate coil. The outer and inner coils are connected 
in series with such polarity that their combined 
mutual inductance to the intermediate coil is 
very nearly zero. The residual mutual induct¬ 
ance is balanced out by the compensators. 

In the sectional view of the search coils, 
Figure 2, the coils may be numbered 1, 2, and 3, 
and the radii and numbers of turns may be 
designated by a lf a 2 , a 3 , and n lf n 2 , n 2 , respec¬ 
tively. If, as in this case, 


Oi _ 02 

a 2 a 3 

then M 12 = kn 1 n 2 a 2 and M 23 = kn 2 n 3 a z , in which 
k is the same constant in both formulas. Letting 


then 


M\2 — M 23 


Ul 03 

n 3 a 2 


The circuit shown in Figure 1 includes a 
1,000-c push-pull oscillator coupled to a pair of 
transmitting search coils, a receiving search 
coil coupled to the input of a two-tube amplifier, 
compensators for reducing to zero the residual 
search-coil coupling, and visual and audible 
output indicators. A test circuit, which includes 
a coil in the field of the search coil, may be 
closed by a push-button and is used to check 
the operation of the unit. The portions of the 
circuit which are grouped in different mechan¬ 
ical divisions are segregated in Figure 1 by 
dot-dash lines. Any metallic body in the field 
of the search coils, which ordinarily have a 
zero coefficient of coupling, couples energy from 
the transmitting coils to the receiving search 
coil. The signal voltage is amplified in the re¬ 
ceiver, and the output is applied to a recti fier- 

b Before undertaking any development work Hazel- 
tine investigated various commercial instruments avail¬ 
able for detecting metal objects. In this investigation 
the Hedden Metal Locators, Inc., a small concern, was 
overlooked. 


which is the condition for balance. 

If a small metallic object, such as a mine, 
approaches the search coils along their axis, the 
resulting coupling between the oscillator and 
receiver coils increases to a maximum at a 
distance slightly less than the radius a 3 of the 
outermost coil. The coupling then decreases, 
becoming zero at the distance a 2 (which is the 
radius of a neutral sphere, i.e., a sphere of zero 
coupling), and increases with opposite polarity 
as the object comes closer. It is intended that 
the search coils be held at least 6 in. above the 
ground when in use in order to reduce the effect 
of the ground; since a 2 is much less than 6 in., 
it follows that a mine will always produce 
maximum coupling when directly under the 
search coils. 

The mutual impedance between the oscillator 
and receiver circuits is adjusted by means of 
two pairs of compensators which are very 
nearly independent. One of each of these pairs 
provides a vernier adjustment. The mutual 


CONFIDENTIAL 




10 


DETECTION OF LAND MINES 


reactance compensator comprises an iron core 
mounted on a screw, permitting it to be moved 
axially through the compensator, the magni¬ 
tude and polarity of the reactance depending on 
the position of the core. The iron core serves to 
increase the flux through the compensator coils 


again depending upon the position of the core. 
It may be noted that residual mutual inductance 
caused by imperfect balance in the search-coil 
assembly is the same for harmonics of the oscil¬ 
lator frequency as for the fundamental fre¬ 
quency. Properly designed compensators must 



in its immediate vicinity with little change in 
the phase of the flux. A brass core moved 
through the resistance compensators adjusts 
the mutual resistance between the circuits. The 
Q of the brass core is much less than 1 (approx¬ 



imately 0.1), so that a component of flux in 
time quadrature with the exciting field is pro¬ 
duced, and mutual resistance is thereby intro¬ 
duced between the oscillator and receiver 
search-coil circuits, the magnitude and polarity 


obey a given relation in order that the 
harmonics and fundamental all balance out 
through the same adjustment. 

Oscillator and Amplifier. A push-pull class-B 
oscillator circuit is used in order to minimize 
the harmonics and, in particular, to cancel the 
second. The receiver search coil is coupled to 
the grid of the first amplifier tube by a step-up 
transformer. Tuning the transformer secon¬ 
dary increases the gain from the receiver coil 
to first grid to a voltage ratio of 93 and provides 
some selectivity. The 1N5GT first amplifier tube 
is impedance-coupled to the output stage by a 
tuned choke. The output tube is a second 1N5GT 
coupled to the earphones and meter by a step- 
down transformer. The output circuit is a high- 
pass filter. An increase in audibility and ad¬ 
ditional selectivity against harmonics and ex¬ 
traneous noise are gained by the use of a 1,000-c 
acoustic resonator. 

General Design Considerations 

The problem of detecting a metallic object by 
a mutual inductance bridge is readily seen to 
be that of detecting the complex impedance 


CONFIDENTIAL 

























































































































































































METALLIC MINE DETECTION 


11 


reflected into the system by a metallic object 
in the presence of other components of complex 
impedance due to the proximity of conducting 
permeable ground. The properties of the soil 
obviously limit the depth at which an object 
can be distinguished from ground effects. Mag¬ 
netic susceptibility and conductivity are the 
soil properties which are important, the dielec¬ 
tric displacement currents being negligible as 
compared with conduction currents. In general, 
there is a borderline frequency below which 
the susceptibility effect is greater and above 
which the conductivity effect is greater. The 
susceptibility effect is independent of fre¬ 
quency 18 while the conductivity effect increases 
with frequency. The soil susceptibility causes 
a change of reactance in the receiver coil, just 
as the object does. The conductivity causes a 
change of both reactance and resistance, the 
resistance change being the greater at lower 
frequencies and the reactance change at higher 
frequencies. All these relations are functions 
of the susceptibility and the depth of penetra¬ 
tion which involve frequency and conductivity. 18 

Electrical Characteristics of the Ground. The 
permeability of average ground was at first 
thought to be near that of free space, but de¬ 
partures from this equality have proved to be 
the greatest cause of spurious responses in 
metal locators. Near one laboratory on Long 
Island the relative susceptibility of the ground 
averaged 0.001, with many lumps of more mag¬ 
netic material. The range of susceptibility 
measurements from these tests varied from 
0 to 0.0025. From the performance of the de¬ 
tectors in the field it is apparent that cer¬ 
tain soils must be more highly magnetic than 
the average mentioned above. For example, 
lava soil and the French pave were particularly 
troublesome in the field. 

The range of conductivity of ground varies 
from practically zero to that of sea water 
(4 mhos per m). The attenuation of the ex¬ 
ploring field due to the conducting ground is 
negligible at a frequency of 1 kc. 

The dielectric constant or specific inductive 
capacity of the ground has been demonstrated 
to be unimportant in the case of metallic mine 
detectors operating at low frequencies. 

The general importance of the electrical char¬ 


acteristics of the ground may be emphasized 
by the statement that for high sensitivity the 
mutual inductance must be neutralized within 
about 1 part in 3,000,000. This corresponds to a 
residual coupling of no more than 0.001 ph. 

System Analysis. The important factors en¬ 
tering into the choice of a system for mine loca¬ 
tion are discussed briefly below: 

1. Target responses. These may be expressed 
in terms of coefficients of resistive and reactive 
mutual coupling produced by metal bodies in 
the exploring field. The mutual impedance re¬ 
flected by the target is expressed by: 

Z = R+jX 
= Lco(k r + jk x ) 

where L is the inductance of a spherical search 
coil. The relative responses and interferences 
may be expressed in terms of the coefficients of 
resistive and reactive response, k r and k x . 

Case 1. Thickness of target greater than 
depth of penetration. At high frequencies, or 
when the depth of penetration of the field in the 
metal of the target is less than its thickness, 
k r ^ar * 10 

Case 2. Thickness of target less than depth 
of penetration. For thin metal shells, k r < k x 
at high frequencies; at low frequency, or with 
very thin metal bodies, k r > k x . Obviously there 
is an intermediate frequency such that k r = k x . 

Case 3. Solid body, radius less than depth of 
penetration. In this case the coefficients are less 
definite because of the distribution of the heat¬ 
ing loss and the stored energy throughout the 
metal. In general, k r > k x . 

The above relations should be considered 
along with reflections from ground in selecting 
optimum operating conditions. 

2. Ground responses. The reflections from 
conductive and/or magnetic ground are consid¬ 
ered mainly for frequencies at which the atten¬ 
uation is negligible at the depth at which the 
target is buried. For this condition low-fre¬ 
quency calculations 18 suffice, and the ground 
effects may be expressed easily in terms of 
analogous coefficients k r ' and k x . The resistive 
coefficient k r ' is found to be proportional to the 
ground conductivity and the frequency. The re¬ 
active coefficient k x is found to consist of two 
terms, one proportional to magnetic susceptibil- 


CONFIDENTIAL 



12 


DETECTION OF LAND MINES 


ity of the ground (which is independent of 
frequency), and a second term which is inde¬ 
pendent of frequency at high frequencies where 
kr predominates. This term varies as the 3/2 
power of the conductivity and of the frequency 
at very low frequencies. The effects indicated 
above are for homogeneous ground. Lumps of 
magnetic material produce positive components 
of mutual reactance and coefficients thereof. 
Ground conductivity reflects substantial inter¬ 
fering components only at high frequencies 
(above about 1 kc over sea water, and above 
about 1 me over ground). The reflections due 
to magnetic earth and rocks are independent 
of the frequency. 

3. Other spurious responses and general com¬ 
ments. The neutralization of the mutual re¬ 
actance between search coils is only as good as 
their mechanical stability. Displacements occur 
either by slow expansion or contraction due 
to temperature changes, or by strains or vibra¬ 
tion. In two-coil structures (overlapping, mutu¬ 
ally perpendicular, etc.) the mutual reactance 
introduced is large and is in direct proportion 
to the displacement. Only small changes remain 
in the triple-concentric-coil construction used 
by Hazeltine in the SCR-625. 28 Even with this 
construction, however, the stability of the 
search-coil mutual reactance limits the sensitiv¬ 
ity which may be used in locators sensitive to 
reflected mutual reactance from the target. 

Within 50 yd or so of power lines, low-fre¬ 
quency locators respond to the magnetic field 
around the line at the fundamental and har¬ 
monic frequencies. The fifth harmonic is often 
large, and locators should be designed not to 
respond to frequencies of 300 c or less. 

Coil arrangements using two equal and op¬ 
posed receiving coils are available which neu¬ 
tralize the response due to uniform fields. These 
reduce greatly the response to power-line noise 
and uniform magnetic soil but produce re¬ 
sponse patterns for the target which have zero 
response in some directions (directly over a 
buried mine) and somewhat reduced response 
in general. 

Screws and other metal parts in search-coil 
mountings cause response in proportion to their 
size and their conductivity or susceptibility. 


Their eifects may be neutralized when they are 
firmly fixed. They should be as far away as 
possible from points where the exploring field 
varies rapidly with small displacements. They 
should be as small as possible and of low con¬ 
ductivity material. They then fall in the cate¬ 
gory of bodies in Case 3 under “Target Re¬ 
sponses.” The resistance component of spurious 
mutual impedance is larger than their react¬ 
ance. 

4. Choice of frequency. The ratio of the re¬ 
sponses from a sizable metal target and from 
conducting ground vary inversely as the 3/2 
power of the frequency. Over ordinary ground, 
the conductivity causes no difficulty at any rea¬ 
sonable frequency. Over sea water, the fre¬ 
quency should probably be less than 10,000 c. 

The reflections from ground caused by its 
magnetic susceptibility are not avoided at any 
frequency, although the effect may be minimized 
in some systems. 

For the simplest locator, 1,000 c is a satisfac¬ 
tory frequency, since simple amplifiers and cir¬ 
cuits and a minimum number of tubes are 
possible. Such a locator can be of the trans¬ 
mitter-receiver type and will give false indica¬ 
tions because of magnetic soil and rocks. For 
locators responsive only to the reflected re¬ 
sistive component from the target, as low a 
frequency as will avoid power-line interference 
is desirable—say 500 to 1,000 c. 

5. Choice of coils. All recommended arrange¬ 
ments require the neutralization of mutual re¬ 
actance between search coils. The search coils 
should be matched to output and input circuits 
by transformers. The concentric three-coil ar¬ 
rangement is recommended for transmitter- 
receiver types where the maximum response is 
desired directly under the search coils. Over¬ 
lapping two-coil arrangements can probably be 
tolerated in devices which are insensitive to 
drift of mutual reactance. 

Special coil structures are available for the 
neutralization of power-line interference and 
uniform magnetic ground effects, but these do 
not have the other advantages of the preceding 
types. Devices which indicate a target by a 
change of frequency are less susceptible to 
interference. This is true, for example, in the 


CONFIDENTIAL 



METALLIC MINE DETECTION 


13 


reception of frequency-modulated radio signals. 

Special field patterns in direction 9 or in depth 
can be developed by suitable coil arrangements. 
For example, the concentric three-coil structure 
and modifications thereof have zero response at 
a distance below the plane of the coils equal to 
the radius of the intermediate coil. 25 The proper 
coil diameter depends on the distance to the 
object. 

Performance 

The performance of the locator in detecting 
the presence of an anti-tank mine on the axis 
of the search coils is shown in Figure 3. The 
target was approximately 8 in. in diameter and 
3 in. high, with superstructure removed. Tests 
in the field indicate that the maximum sensi¬ 
tivity of the locator is greater than can be used 
in the presence of rocks having appreciable 
magnetic susceptibility. Under these conditions 
the response to a mine which is buried a foot 
deep may become indistinguishable from the 
response to rocks nearer the surface. The aver¬ 
age susceptibility of the soil during the above 
tests was of the order of 0.001 relative to free 
space. 


Phase Discrimination Locators 

After the design of the SCR-625 was frozen, 
it became apparent from further experimental 
work at Hazeltine that a circuit which discrim¬ 
inated against reflected complex impedance 
would be, in general, superior for practical use. 
Such a circuit could reduce or almost completely 
remove false indications and high-level back¬ 
ground noise from magnetic ground and rocks. 
It was recognized that such a phase discriminat¬ 
ing system would probably not be quite as satis¬ 
factory for the detection of certain small metal¬ 
lic objects in which the induced eddy-current 
effect is small. For large bodies, however, this 
system, which shifts the oscillation frequency 
by reflected mutual resistance, showed great 
possibilities. In fact, at the present writing, the 
phase discrimination type of mutual inductance 
detector is the most advanced development in 
metallic mine detection. A brief description of 


a typical system will be given in this section as 
representative of the “state of the art.” 

The Problem 

Experience with various 1,000-c locators of 
the simple transmitter-receiver type indicated 



Figure 3. Locator performance in the detection 
of a mine on the axis of the search coils. Sensitiv¬ 
ity control set to give 200-/ua output with test 
switch closed and with full meter sensitivity. 


two inherent weaknesses: (1) they are sensi¬ 
tive to slightly magnetic rocks or soil and to 
drift in mutual inductance between the coils ; 
(2) both drift and magnetic ground effects pro¬ 
duce coupling components which are nearly 
pure mutual inductance. These residual re¬ 
sponses must be neutralized to the order of 
1 part in 1,000,000 for operation at high sen¬ 
sitivity levels. It was thus apparent that a 
system which responded differently to the mu¬ 
tual resistance reflected by a metal target than 
to mutual reactance from any source would go 
far toward alleviating magnetic soil and drift 
objections. 

History 

Such a phase discriminating circuit, for an 
audio-frequency detector, was developed 30 by 
an NDRC contractor, the Hazeltine Service 
Corporation, Little Neck, L. I., in the spring of 


CONFIDENTIAL 























14 


DETECTION OF LAND MINES 


1942. This was a few months after the stand¬ 
ardization for production of the SCR-625. Two 
to three years later, after the magnetic ground 
difficulty had proved to be a serious problem 
in combat use, contractors of the Engineer 
Board developed phase discrimination systems 
in order to solve the same problem. These latter 
systems were not completed in time to be placed 
in production as a replacement detector for 
the SCR-625 before the end of the Japanese 



R t + j x^. 

Figure 4. Block diagram. 


war. It thus appears as a direct consequence 
of the cessation of development work by NDRC 
on solely metallic mine detectors (none of which 
was done after the spring of 1942) that no 
improved metallic mine detector making use of 
the phase discrimination principle reached the 
field. 

One reason for this unfortunate result was a 
policy decision on the part of NDRC, in agree¬ 
ment with then existing Army policy, that fur¬ 
ther development of a metallic mine detector 
based on the mutual inductance principle was 
an engineering problem, rather than a research 
problem, and therefore should properly fall 


under the cognizance of the Army. The failure 
of the Army further to explore this principle 
may in part have been due to a lack of famili¬ 
arity with the work of NDRC, which, in turn, 
may have been caused in part by a change in 
personnel responsible for the NDRC develop¬ 
ment program. 

Method of Operation and Circuit Design 

A brief summary of the phase discriminating 
circuit developed under NDRC contract is pre¬ 
sented here as one possible solution of the prob¬ 
lem. Reference should be made to the reports 
on metallic mine detectors by contractors of the 
Engineer Board for other circuits designed to 
produce the same results. 

The frequency selected for the device was 
1,000 c, a near optimum frequency for the 
detection of bodies of reasonable size because 
it eliminates effects caused by ordinary ground 
conductivity. The mutual resistance due to the 
target is made to shift the frequency of an 
oscillating circuit. The frequency change is ob¬ 
served by comparing the shifted frequency with 
a fixed frequency by the beat method. The dif¬ 
ference frequency is then detected by a set of 
earphones. The phase shift is so adjusted that 
mutual reactance maintains the amplifier in 
oscillation and affects only the amplitude of the 
oscillation. 

The feedback which produces oscillation is 
obtained by a small amount of controlled posi¬ 
tive mutual reactance between the search coils. 
The total voltage induced in the receiving 
search-coil circuit is the vector sum of that due 
to the controlled mutual reactance and that 
due to the mutual reactance and mutual re¬ 
sistance of the target. Since the feedback circuit 
can oscillate only with a net phase shift of zero, 
the frequency of oscillation changes until the 
sum of the phase shift in the amplifier and the 
net mutual impedance between the search-coil 
circuits is zero. 

A block diagram of the circuit is shown in 
Figure 4 and the complete circuit in Figure 5. 
The manual mutual resistance R 0 is used only 
to balance out residual mutual resistance in the 
circuit in the absence of a target; therefore, 
the net initial mutual resistance is zero. The 
phase corrector is used for fine adjustments of 


CONFIDENTIAL 








































METALLIC MINE DETECTION 


15 


phase so that the initial frequency of the cir- not limited to the design used with the SCR- 
cuit is the same as that of the fixed oscillator 625; their sizes should be chosen with reference 
(fo). Alternatively, a frequency adjustment of to the distance at which targets must be de- 



the independent oscillator might be used. Once 
the frequency is corrected, only infrequent re¬ 
adjustments are required. 

Coil structures for use with this circuit are 


tected. Their positions (concentric, mutually 
perpendicular, etc.) should be chosen with ref¬ 
erence to the desired distribution pattern of 
their exploring fields. 9 


CONFIDENTIAL 








































































































































16 


DETECTION OF LAND MINES 


Performance 

Extensive performance tests were not car¬ 
ried out with this circuit; the following results, 
therefore, are far from complete. A dummy 
8-in. anti-tank mine, buried 23 in. below the 
surface, was detected easily in magnetic ground 
with the search-coil disk 6 in. above the ground; 
it could just be detected with the search coil 
12 in. above the ground. By way of contrast, it 
was necessary to adjust and carry the search 
coils of a SCR-625 locator at a critical height 
of about 8 in. above the ground in operation; 



Figure 6. Disassembled search unit. 


the mine could then be detected with great care 
at a depth of 18 in. in the same soil. No mag¬ 
netic rocks were indicated by the new locator, 
whereas the SCR-625 detected many. A pow¬ 
dered-iron ring 1% in. in diameter did not 
change the frequency of operation at any dis¬ 
tance from the search coil. Although this ring 
has small mass, it produced definite indications 
with the SCR-625 locator at a distance of about 
lft. 


1,3,5 The Portable Iron Detector and 
the AN/PSS-1 

The portable iron detector 1 is an extremely 
simple device for detecting ferromagnetic ob¬ 
jects buried in the earth by means of their dis¬ 
torting effect on the earth’s magnetic field. Its 
major practical importance is derived from 
the fact that it was the forerunner of the 
AN/PSS-1, a detector designed for use by 
swimming members of naval combat demolition 
teams for the detection of anti-boat mines. The 
PID was developed under an NDRC contract 
by the Department of Terrestrial Magnetism, 
Carnegie Institution of Washington, while its 


modification to the AN/PSS-1 was carried out 
under a contract with the Engineer Board. 

In principle the PID is a space-time gradi- 
ometer consisting of two magnetometers 
aligned with each other but with their outputs 
connected in opposition. In this arrangement 
the effect of the uniform magnetic field of the 
earth is cancelled out, while the effect of a non- 
uniform magnetic field, such as that resulting 
from a ferromagnetic object magnetized by 
induction, yields a signal that may be amplified 
and made audible. Thus the application of the 
gradiometer principle permits the detection of 
a small anomaly in the presence of a large, 
uniform magnetic field. 

The search unit (see Figure 6) consists of two 
Permalloy rods 3/16 in. in diameter and 6 in. 
long, mounted so as to have a common axis and 
a spacing between centers of about 8 in. Sur¬ 
rounding each of these rods is a coil of 40,000 
turns of No. 40 insulated copper wire. The two 
coils are connected in series opposition with 
the terminals brought out to an amplifier and 
indicator. The resultant mine signal from the 
detector head is a weak, unidirectional emf hav¬ 
ing a duration of the order of one second. 

Figure 7 shows the instrument in operation. 

According to the circuit diagram of Figure 8, 
an emf developed in the search unit passes 
through two stages of amplification and is used 
to modulate the output of a 1,000-c phase-shift 
oscillator. The amplified signal voltage and car¬ 
rier are applied to the grids of the modulator 
tubes in such a manner that the modulator out¬ 
put contains only the signal frequency and the 
side frequencies. Since the signal frequency is 
very low and the carrier frequency is between 
800 and 1,000 c, the side frequencies are nearly 
the same as the carrier frequency. Further¬ 
more, the side-frequency amplitudes are directly 
proportional to the amplitude of the signal 
frequency. The output of the modulator tubes 
is passed through one stage of audio amplifica¬ 
tion and fed into headphones through a match¬ 
ing transformer. Distinct pulses of a high- 
pitched tone are heard by the operator as the 
search unit passes near a magnetic object. 

Factors limiting the usefulness of the PID 
are: (1) lack of sufficiently precise balance and 
alignment of the two rod-coil combinations, 


CONFIDENTIAL 








METALLIC MINE DETECTION 


17 


(2) insufficient elimination of rotations of the 
pickup device during a sweeping motion, and 

(3) the lack of an entirely satisfactory means 
of amplifying the very small, slowly changing 
emf developed in the search unit. The sensi¬ 
tivity of the PID to ferromagnetic objects is 
approximately equivalent to that of a mutual 



Figure 7. Search unit in operation. 

inductance detector, such as the SCR-625; it is, 
of course, of much simpler design. Certain 
modifications of the PID, if carried out, would 
probably increase its range beyond that of a 
mutual inductance detector. 

The AN/PSS-1 is similar to the PID in all 
fundamentals. Engineering changes have been 
incorporated, however, such as mechanical ro¬ 
tation of the magnetometer coils and complete 
waterproofing. It is of interest to note that the 
Permalloy cores or rods of the PID were re¬ 
moved in the AN/PSS-1. This permitted an 
increase in the stability of balance, resulting 
in a net increase in useful sensitivity over that 
with the rods in place. 

The AN/PSS-1 is able to detect anti-boat 
mines, such as the Japanese J-13, at a range of 
about 2 ft under water. Careful magnetic 
gradiometer tests with other instruments in¬ 
dicate that the anomaly due to the presence of 


such a mine does not extend farther from the 
mine. 


1,3,6 Evaluation of Metal-Detector Program 

The design and construction of locator equip¬ 
ment for metallic mines was, for the most part, 
a straightforward engineering research prob¬ 
lem to which there are now a number of satis¬ 
factory solutions that vary more in detail than 
in fundamental principle. At the end of slightly 
less than a year of development work a locator, 
the SCR-625, was standardized and in the 
spring of 1942 was placed in production. This 
instrument was highly satisfactory except 
where magnetic ground was encountered, when 
its usefulness was seriously impaired. As this 
limitation was recognized in the laboratory 
prior to field usage, it might be expected that 
within a year or so production of the original 
detector would be supplanted by that of a modi¬ 
fied locator of improved characteristics. This 
would be the natural chain of events if develop¬ 
ment work kept pace with field requirements. 
However, no basic experimental development 
work on its improvement was carried out in the 
year and a half following its standardization. 
When a project for improvement of metal lo¬ 
cators was taken up at a later date by the Engi¬ 
neer Board, the continuity of the original de¬ 
velopment program had been lost, with the 
consequence that other contractors were forced 
to rediscover the design principles which had 
previously been clearly established. This loss of 
time accounts in the main for the fact that no 
detector other than the SCR-625 got into field 
use from production lines in the United States. 

At the present writing it is believed that de¬ 
sign considerations for a metal locator are well 
worked out. This statement applies particularly 
to locators based on the mutual inductance 
bridge principle and circuits utilized therewith 
for discriminating against spurious reactive 
signals caused by magnetic ground. 

Considerably less attention has been devoted 
to the development of certain special-purpose 
equipments which have possible military appli¬ 
cations; for example, the beach detector, basi¬ 
cally a metal detector, can probably be im- 


CONFIDENTIAL 




18 


DETECTION OF LAND MINES 


proved very easily in the light of our present 
knowledge. It is also probable that a number of 
engineering advances can be made in designing 
equipment for the detection of ferromagnetic 
objects. 

As a summary, it may be stated fairly that 
the technical side of the metallic mine locator 
problem was handled adequately by a number 
of contractors; looking back, however, it seems 


This expectation was verified by the trend in 
enemy mine warfare. The use of fewer metal 
parts by the enemy in the construction of its 
mines could be explained not only by the sup¬ 
position that certain metals were becoming in¬ 
creasingly scarce and by production consider¬ 
ations but also by crediting them with the 
attempt to reduce the effectiveness of metallic 
mine detectors. Fortunately, it turned out that 


BALANCED 

LOW-FREQUENCY AMPLIFIER MODULATOR PUSH-PULL AMPLIFIER 



clear that the timing of the program could have 
been improved. 


14 NONMETALLIC MINE DETECTION 

Early in the war it was recognized that large- 
scale enemy employment of nonmetallic mines 
would almost completely negate the usefulness 
of standard metallic mine detectors. The Ord¬ 
nance Department, ASF, was actively engaged 
in the development of a nonmetallic mine for 
use by the Allies, and it was natural to assume 
that the enemy was following similar lines. 


only toward the close of the European campaign 
did the Germans actually introduce completely 
nonmetallic anti-tank and anti-personnel mines 
in quantity. 

In this chapter the NDRC program on the 
development of nonmetallic mine locators is de¬ 
scribed. Although classified here as nonmetal 
detectors, every detector able to locate non¬ 
metallic mines will also locate most metallic 
mines under the proper circumstances; to this 
extent these detectors might be classified quite 
appropriately as universal mine detectors. This 
nomenclature is not followed in this report for 
two reasons: (1) a separate treatment empha- 


CONFIDENTIAL 








































































NONMETALLIC MINE DETECTION 


19 


sizes the nonmetallic aspects of the problem; 
(2) the detectors described are far less success¬ 
ful in locating metallic mines than standard 
metallic mine detectors. 

The development of a nonmetallic mine lo¬ 
cator was officially requested by the Army 
Engineers under project CE-31 in February 
1943. At the time of acceptance no certain 



Figure 9. AN/PRS-1 detector in operating 
position. 


method of detecting nonmetallic mines or ex¬ 
plosives as such buried in the ground was 
known. Since the problem was recognized to be 
of high priority by Section 17.1, it adopted the 
policy of exploring all methods which appeared 
to offer any promise of success. These are listed 
in the following paragraph. 


Possible Operating Principles 

1. Seismic methods employing mechanical vi¬ 
bration in the ground under test are based on: 
(a) variations in the mechanical impedance of 
the ground, (b) variations in the mechanical 


transmission characteristics of the ground be¬ 
tween two points, (c) reflection of supersonic 
waves from buried solids to the soil surface, 
and (d) variation in the earth’s “tone” pro¬ 
duced by light sharp blows against the ground. 

2. Electrical methods are based on the elec¬ 
trical properties of the soil, consisting of: (a) 
variations in the conductive symmetry of the 
soil with respect to a normal to the ground, 
(b) variations in electromagnetic and electro¬ 
static fields induced in the soil, and (c) varia¬ 
tions in reflected waves at ultra-high frequen¬ 
cies. 

3. Radioactivity methods are based on: (a) 
variations of the natural radioactivity of the 
earth due to the presence of a buried object, 
(b) variations in neutron- and y-ray emission 
from the ground when it is bombarded by a 
neutron source, and (c) variation in the scat¬ 
tering of y-rays from the ground when it is 
illuminated by a y-ray source. 


1,4,2 Military Requirements 

Initial military characteristics laid down for 
the detection of nonmetallic mines were neces¬ 
sarily vague and general. The only perform¬ 
ance characteristic mentioned was that the de¬ 
tector should detect reliably all nonmetallic 
anti-tank mines. It was thought at one time that 
the detector should be able to detect a 5-lb mass 
of TNT buried 6 in. deep, but this requirement 
was later discarded as too severe. Military 
characteristics further required that the device 
should be portable, simple, and capable of 
being operated by a man standing, kneeling, 
or in a prone position. It was stipulated that 
the weight of the exploring rod should not ex¬ 
ceed 10 lb and that of the amplifier and other 
accessories should not exceed 20 lb. Both aural 
and visual methods of indication were desired. 

For practical field use very severe military 
requirements exist for a suitable mine detector, 
either metallic or nonmetallic. The nonmetallic 
mine detector should, it turns out, be able to 
detect reliably (meaning practically 100 per 
cent of the time under all conditions) both anti¬ 
personnel and anti-tank nonmetallic mines at 
operational depths of burial. Furthermore, op- 


CONFIDENTIAL 








20 


DETECTION OF LAND MINES 


eration of the detector must be faster than that 
of manual probing methods. It should be pos¬ 
sible to operate the detector at night under 
enemy surveillance. 


14 ' 3 Development of the AN/PRS-1 

Early in 1943 a prospectus describing possi¬ 
ble ways of locating nonmetallic mines was sent 
to the Engineer Board by RCA-Victor Division, 
Indianapolis, Indiana. This was referred to 


ance resulting from this work have not been 
gratifying. 

Description of Instrument 

The AN/PRS-1, as developed by RCA-Victor, 
comprises a detector head (see Figure 9) con¬ 
sisting of an antenna structure (radiator and 
reflector) and a U-H-F oscillator mounted on 
a sectionalized handle, and a pack (worn on 
the operator’s back) which contains batteries 
and audio circuits. The U-H-F oscillator, based 
on a type 955 acorn triode, is an ultra-audion 



Figure 10. AN/PRS-1 detector—schematic diagram, oscillator, and amplifier. 


NDRC, together with a request that the sugges¬ 
tions be explored and evaluated. After investi¬ 
gation, NDRC awarded a contract to the com¬ 
pany, and by the end of the year a satisfactory 
prototype model of a detector, later known as 
AN/PRS-1, had been developed and given lim¬ 
ited tests. Since that time considerable work 
has been expended in attempting to improve 
the AN/PRS-1 by engineering modifications 
and by more fundamental variations in general 
design features. The improvements in perform- 


type. The feedback ratio is determined by the 
tube interelectrode capacitances, which also act 
as tuning capacitance. An external adjustable 
inductance between plate and grid provides 
frequency control and feeds energy at about 
300 me to the dipole antenna by inductive 
coupling. When the detector head is held close 
to the ground, as it is when in use, the ground 
acts as an r-f load which appears as a change 
in the antenna impedance. Changes in the im¬ 
pedance of the soil thus appear as changes in 


CONFIDENTIAL 


































































































NONMETALLIC MINE DETECTION 


21 


the oscillator loading, and are indicated on a 
grid current microammeter. If the oscillator is 
operated with enough loading almost to stop 
oscillation, the grid current becomes sensitive 
to soil impedance. In addition to the meter, a 
tone indication in a headset or resonator is 
produced by amplitude-modulating a 1-kc sig¬ 
nal with the grid bias developed by the r-f tube. 
A schematic diagram of the wiring circuit is 



Figure 11. Equivalent high-frequency circuit. 


shown in Figure 10, and the equivalent high- 
frequency circuit is shown in Figure 11. 

Design Considerations 

Relationship of Penetration Depth to Fre- 
qency. Attenuation calculations 33 ' 36 indicate 
that there is practically a constant depth of 
penetration of the ground for frequencies in 
excess of 10 me. These computations also indi¬ 
cate that for a given frequency the penetration 
(and, therefore, the sensitivity) of a detector 
decreases with increasing ground conductivity. 

Reflection Effects. The detector produces a 
steady-state U-H-F oscillation that causes 
standing waves in the earth and in the air be¬ 
neath the antenna. It is, therefore, to be ex¬ 
pected that the sensitivity of detection will be 
a cyclic function of the height of the detector 
above the ground and a cyclic function of the 
depth of burial of the mine. Depending on the 
frequency of operation (for example, at 1,260 


me) there may be several heights within the 
normal operating range at which the sensitivity 
is nearly zero. The number of such nulls in¬ 
creases with frequency. The cyclic variation 
of sensitivity with depth of burial, or depth 
function, is dependent on the electrical charac¬ 
teristics of the soil, the frequency, and the 
electrical characteristics and dimensions of the 
buried object. These effects have been computed 
using Maxwell’s equations and have been veri¬ 
fied experimentally, as shown in Figure 12 and 
Figure 13. With reference to these figures, sen¬ 
sitivity to objects is registered by the oscillator 
as variations greater or less than the average 
grid current. Points of insensitivity then are 
spaced one-quarter wavelength apart, or 1.09 
in. in water at 300 me. In Figure 13, M-5 refers 
to the U. S. Army M-5 nonmetallic anti-tank 
mine, which is constructed of three general 
materials: air, glass (case), and TNT explosive. 
In the depth function a null is found on the 
surface, and peak sensitivity of detection is 
obtained at one-quarter wavelength below the 
surface. Propagation velocity, frequency, and 
wavelength in any medium are, of course, re¬ 
lated by the formula 



in which / is frequency, A is wavelength, and v 
is phase velocity. 

The velocity in terms of the electrical char¬ 
acteristics of the medium is given by 

where /x is permeability and k is the dielectric 
constant. 

Resolution. In order to obtain the maximum 
indication from a mine, it is necessary to have 
the electric length of the dielectric boundary 
approximately one-half wavelength or greater, 
as measured in the medium of least dielectric. 
This is shown experimentally in Figure 14. 
This length must be parallel to the dipole an¬ 
tenna because of polarization. From optical 
theory, and also from the aforementioned ex¬ 
periments, it would be expected that the resolu¬ 
tion of a detector system should increase with 


CONFIDENTIAL 



























22 


DETECTION OF LAND MINES 


frequency. This has been verified, as will be 
mentioned later. 

Conclusions in Determining 
Frequency of Operation 

1. Frequency relation to mine size. Since the 
initial performance characteristics called for 
detection of anti-tank mines, the M-5 nonmetal- 
lic mine was chosen as a representative sample 
of the type of object it was desired to detect. 


2. Frequency relation to penetration depth. 
This consideration does not restrict the fre¬ 
quency choice too much; as has been previously 
noted, the penetration remains nearly constant 
for frequencies above 10 me. 

3. Frequency relation to null-point depth. 
Ground conditions are quite variable, the dielec¬ 
tric constant varying from 5 to 25, whereas 
that for pure water is 81. Peak sensitivity of 
detection will be obtained at 1/4A below the 



The properties of the materials in this mine 
and its dimensions are listed in Table 1. In this 
mine, air and glass, because of their thickness, 
do not produce as much reaction as TNT, which 
is detected quite easily at 304 me. The use of this 
frequency tends also to eliminate detection of 
smaller objects, such as rocks and air pockets, 
which if detected increase the number of false 
indications. 0 

c It will be noted here that the initial design of the 
AN/PRS-1 was directed particularly toward the de¬ 
tection of large nonmetallic mines, not toward the de¬ 
tection of small anti-personnel mines, such as Schu 
mines, which later became most important in the Ger¬ 
man theaters of operation. 


surface. A practical depth for a mine is 3 in., 
and it therefore follows that a A of 12 in. is 
desired in the soil. An average dielectric soil 
condition is between 10 and 15, indicating the 
use of a frequency between 310 and 254 me. 

4. Summary. Depth function considerations 
designate a frequency which is compatible with 
that chosen (for the M-5 mine) from resolution 
considerations; these, in turn, are compatible 
with the penetration effects. There remains to 
be introduced the cyclic variation in detector 
height or height function. It may be argued, in 
general, that increasing the frequency of oper¬ 
ation because of the standing-wave patterns 


CONFIDENTIAL 












































































NONMETALLIC MINE DETECTION 


23 


increases the number of nodes per unit distance 
the detector is held above the ground. It would 
be expected that increasing the frequency 
would, therefore, make it more difficult to com¬ 
pensate for variations in detector height during 
normal mine-sweeping operations. However, by 
certain design tricks, 33 it was possible to in¬ 
troduce some height compensation into the 



01 23456789 10 11 12 


HEIGHT ABOVE GROUND IN INCHES 

Figure 13. Observed data of height and grid 

current. 

AN/PRS-1, giving it a fairly flat characteristic 
at the normal operating height of 4 in. This com¬ 
pensation is difficult to regulate and is much 
more satisfactory over dry soil than over wet 
soil. Thus a fairly satisfactory control of height 
function can be achieved at 300 me. 

Performance 

There are numerous deficiencies in the per¬ 
formance of the AN/PRS-1, some of which 
can be anticipated from the foregoing discus¬ 
sion. In general terms these deficiencies or limi¬ 
tations may be listed as follows. 

False Indications. In operation false indica¬ 
tions were encountered because of insufficient 
height compensation. Considerable care was 
necessary in sweeping with the detector in 
order to prevent variations more than an inch 
or two in height above ground; over uneven 
terrain such constancy of height was frequently 
impossible to achieve. More important sources 
of false indications were roots, stones, recently 
disturbed patches of earth, etc.—macroscopic 
changes in the electrical characteristics of the 
ground which the detector was unable to dis¬ 
criminate against. 

Reliability. Except under certain circum¬ 


stances, the AN/PRS-1 was a reliable detector 
of both metallic and nonmetallic anti-tank 
mines. Its sensitivity to metallic mines was 
considerably less than that of a standard mine 
detector, such as the SCR-625. It was unreliable 
in its detection of small anti-personnel mines, 
e.g., the Schu mine, which it was not designed 
to detect, and the German S mine. Since detec¬ 
tion sensitivity is a function of the depth of 
the mine below the surface of the ground, under 
some conditions sensitivity minima can be 
found at depths typical of actual mine-laying 
practice. In addition, the AN/PRS-1 performed 
unsatisfactorily over extremely dry ground 
(such as dry sand) and over extremely wet 
ground. 

Mechanical Design. The above objections 
are inherent in the electric system of the 
AN/PRS-1. Other criticisms of the PRS-1 were 



5 10 15 20 25 30 35 40 45 50 55 


LENGTH OF CAVITY IN CM 

Figure 14. Observed data of air-cavity reflection 
in ground. 

based on its vulnerability to mechanical shock 
and certain engineering design features which 
were not basic limitations. 

Miscellaneous. Miscellaneous objections or 
limitations include a polarization effect and the 
necessity for frequent readjustment. Dipole 
radiation is strongly polarized, thus tending 
to detect only objects with a dimension parallel 
to the antenna of 4 in. or larger. Nodal points 
shift greatly because of varying ground con¬ 
ditions, which may be caused by changes in 


CONFIDENTIAL 
































































24 


DETECTION OF LAND MINES 


the moisture content affecting the dielectric 
constant of the soil. Adjustments must be made 
to compensate for these variations. 

Advantages of this detector are its ability 
to operate by being swept over the ground like 
a metallic mine detector, the simplicity of its 
circuit, and its ability to detect both metallic 
and nonmetallic mines. It is perhaps worth 
mentioning that the AN/PRS-1 is believed to 
be the only nonmetallic mine detector produced 
by any of the combatants in World War II, 
although attempts were made by other coun¬ 
tries to design such a detector. It could not 
supplant the SCR-625 operationally because 
most of the enemy mines contained some re¬ 
sidual metal, which could usually be detected 


of the nodes of detection sensitivity with fre¬ 
quency. It appeared that if a detector were 
made responsive at more than one frequency, 
the nodes of sensitivity could be staggered; 
and when one frequency was insensitive, an¬ 
other might be at maximum sensitivity. Both 
RCA-Victor Division, New York and another 
contractor, Polytechnic Institute of Brooklyn, 
N. Y., used this as a basis for one general attack 
on the problem of developing an improved 
locator. Further consideration of this method 
led to the conclusion that a higher frequency 
would be needed if the resolution was to be 
increased appreciably. 

Early efforts thus were directed toward the 
development of facilities suitable for investi- 


Table 1 . Properties of materials comprising U. S. M-5 nonmetallic mine. 


Materials 

Diam¬ 

eter 

(in.) 

Thick¬ 

ness 

(in.) 

Di¬ 
electric 
Constant 
( k) 

Velocity 

of 

propagation 

Frequency 
corresponding 
to %X of diam. 

Thickness 
corresponding 
to / (me) 

Air space 

10 

1 

1 

3.0 x 10 8 m per sec 

590 me 

1/20 \ 

Glass case 

10 

1 

6.2 

1.2 x 10 8 m per sec 

236 me 

1/20 X 

TNT explosive 

10 

3 

3.75 

1.55 x 10 8 m per sec 

304 me 

1/6 X 


by a metal locator, and because hand probing, 
though slow, was almost 100 per cent reliable 
as a means of detection. 


Other U-H-F Detectors and 
Improvements in the AN/PRS-1 

As summarized in the last section, there 
were certain limitations characteristic of the 
AN/PRS-1 which prevented it from becoming 
a useful detector in the field. Logically the next 
step in the development was to reduce or elim¬ 
inate design deficiencies so far as possible. 
Attempts were made not only to increase per¬ 
formance but also to reduce weight, improve 
ruggedness, and to make other engineering de¬ 
sign advances. Toward these ends new U-H-F 
systems were investigated, and modifications 
of the AN/PRS-1 system were considered. 

The cyclic nature of the height function and 
the depth function, explained as standing-wave 
effects, has already been mentioned, as has the 
expected variation in the number and positions 


gating detector characteristics at a number of 
frequencies, all higher than 300 me. First at¬ 
tempts made use of the technique of continu¬ 
ously sweeping over a broad frequency spec¬ 
trum. Results were somewhat obscured by 
limitations in the apparatus and technique. 
Later tests were made at spot frequencies. The 
results of these tests led to a study of detection 
characteristics under carefully controlled con¬ 
ditions at a single high frequency. Analysis 
of results up to this point 35 gave a new per¬ 
spective: the importance of the depth function 
appeared to be minimized, and the problem of 
height compensation stood out as the major 
objective. 

A study of detection characteristics at 1,270 
me confirmed earlier concepts and led to the 
development of a detector consisting of a source 
of radiation directed toward the earth and a 
directive receiver which responded only to sig¬ 
nals reflected from buried objects. This ap¬ 
proach was carried on at various times 35 inde¬ 
pendently by the two contractors. 32 The method 
was known commonly as the Brewster angle 


CONFIDENTIAL 







NONMETALLIC MINE DETECTION 


25 


method from the analogy to the optical case, 
and was studied both at 10-cm and 3-cm wave¬ 
length (S band and X band). 

In addition to this program, RCA-Indianap- 
olis continued development work using approx¬ 
imately the same operating frequency. Problems 
included not only increased performance and 
less weight, but also (1) more precise specifica¬ 
tions for elements of the AN/PRS-1, (2) ade¬ 
quate protection for the antenna, (3) a labora¬ 
tory method of determining and measuring the 
sensitivity of the detector, and (4) a more 
easily adjusted detector. 

The program described above resulted in the 
construction of many laboratory detectors 
which were discarded and replaced by other 
models as better (or at least newer) ideas were 
formulated. In the remainder of this section, 
attention will be confined mainly to a descrip¬ 
tion of the Brewster angle detector [BAD] and 
an evaluation and summary of the results from 
other investigations. The BAD is discussed 
separately because, although its development is 
not completed and therefore its limitations are 
not well known, it appears to have application 
as a vehicle-mounted detector. Final models of 
the AN/PRS-1 type are only briefly mentioned; 
a more complete description of them is made 
in the next chapter under universal or com¬ 
bination detectors, as these models do not ap¬ 
pear to have an application by themselves. The 
two-frequency scheme already mentioned will 
not be discussed further other than to state 
here that it was tried and discarded inde¬ 
pendently by the two different contractors for 
the following reasons: (1) at practical fre¬ 
quencies ( S - and X-band ranges) reflections 
from surface irregularities could not be elim¬ 
inated 32 ’ 35 or minimized sufficiently; (2) be¬ 
cause of high attenuation, the depth of pene¬ 
tration was inadequate for practical use in 
conductive soils. 

AN/PRS-1 Detector Advances 

Although extension of the frequency range 
by a factor of ten higher did not lead to 
a detector superior in performance to the 
AN/PRS-1, further investigations at or near the 
PRS-1 frequency (300 me) resulted in models 
definitely superior to it. The NDRC versions of 


this advance were designated by the Engineer 
Board as detector sets AN/PRS-1 (XB-2) and 
(XB-3). Twenty-five units of each of these 
detectors were to be tested by the Engineer 
Board and the Engineer School. It was the opin¬ 
ion of NDRC that these detectors did not repre¬ 
sent a really fundamental advancement in the 
art of detection and that there was little likeli¬ 
hood of further engineering changes in the 
basic U-H-F system resulting in the develop¬ 
ment of a universal detector capable of satis¬ 
factory performance in the field—unless uti¬ 
lized in combination with a metal detector. 
(Detectors of this type will be described in the 
next section.) This viewpoint may be divergent 
from that held by the Engineer Board. A con¬ 
tractor of the Board, who studied the 
AN/PRS-1 system, has developed a detector 
set (AN/PRS-4) similar to it which the Board 
feels has a good chance of being a satisfactory 
universal mine detector. Field tests will un¬ 
doubtedly settle the question in the near future. 

Brewster Angle Detector 

Prior to the work on the BAD the Polytechnic 
Institute of Brooklyn, which developed the de¬ 
tector described in this section, attempted a 
number of other solutions, but each had 
faults equal to or greater than those of the 
AN/PRS-1. The methods 32 tried were: (1) 
radiating and receiving two frequencies with 
a single rectangular horn; (2) radiating one 
or two frequencies from one rectangular horn 
and receiving on another; (3) radiating simul¬ 
taneously from two horns and receiving on a 
small antenna between them; (4) radiating 
from a biconical horn and receiving on a small 
antenna at the axis of the horn; (5) transmit¬ 
ting signals horizontally through the ground 
by means of a buried antenna or by means of 
a projecting horn, and receiving the scattered 
energy from the ground. Thus the contractor 
had considerable experience with the general 
problem in the wavelength range of 10 cm 
before initiating studies of the BAD. 

The Brewster angle method is based on the 
following well-known property of electromag¬ 
netic waves: if there be incident upon a bound¬ 
ary between two dielectric media an electro¬ 
magnetic wave polarized with the electric vector 


CONFIDENTIAL 



26 


DETECTION OF LAND MINES 


in the plane of incidence, there is a critical 
angle of incidence at which the waves will be 
totally refracted so that no reflection from the 
surface of one medium occurs. Actually, at this 
critical angle the reflected wave exists to the 
extent that, mathematically, it travels down the 
boundary layer between the two media, but 
there is no energy associated with this wave. 
It was felt that if a transmitted wave could 
be totally refracted into the soil, there would 
be no interfering signal above the ground, 
caused by ground reflection, to mask a mine 
signal. In the presence of a mine or other dis- 



Figure 15. Parabolic cylindrical antenna. 


turbing object buried beneath the surface, the 
refracted ray would be partially reflected, this 
ray in turn being refracted at the ground sur¬ 
face into a properly placed receiver. 

Apparatus Description. Photographs of the 
head of the BAD in its latest developmental 
stage are shown in Figures 15 and 16. The 
transmitting antenna, which is connected to a 
U-H-F oscillator, comprises an array of dipoles 
placed on the focal axis of a parabolic cylin¬ 
drical reflector. This method of antenna con¬ 
struction was used in an attempt to obtain 
nearly plane waves of radiation so that the area 
illuminated on the ground would be roughly 
the projection of the reflector area. A similar 
antenna array is used in the detector head. A 
portable microwave power source was not de¬ 
veloped during the course of the contract, the 
power source used being a Sperry type 411 
klystron, pulse-modulated by a simple blocking 
oscillator. A clipper amplifier in the detecting 


circuit was designed to eliminate small spuri¬ 
ous signals which were found to be present 
even under the best conditions. This amplifier 
is so designed that these small signals are over¬ 
ridden by using a pulse modulation of the high- 
frequency source and by using an amplifier 
following the crystal receiver which would not 
be responsive to signals of low level. The cir¬ 
cuit diagram of a semiportable amplifier of an 
experimental design which fulfilled the above 
requirements is shown in Figure 17. 

Since total refraction takes place at such 
large angles (75 to 60 degrees) of incidence, 
direct coupling resulted between the transmit¬ 
ter antenna and the receiver antenna. To bal¬ 
ance out this direct signal, a system of com¬ 
pensation was developed, effected by feeding 
the signal from the transmitter directly into 
the receiving antenna by a separate coaxial line. 
A special feedback device was constructed to 
enable adequate adjustment of amplitude and 
phase of the auxiliary balancing signal fed into 
the receiver. 

Performance. Extensive tests with this de¬ 
tector in a box filled with sand of varying 
degrees of moisture revealed it would unerr- 



Figure 16. Brewster’s angle device. 

ingly locate a large nonmetallic mine or a 
small metallic one. Moreover, it did not respond 
at all to irregularities of the surface of the 
soil, to variations of height above the sand, or 
to variations in tilt. Its only drawbacks ap¬ 
peared to be that it would not respond to a small 
nonmetallic mine, and it would not respond 
to a mine which had been planted with its flat 
surface far from parallel with the surface of 
the soil. Further tests may prove this latter 
objection to be extremely serious, because it 


CONFIDENTIAL 








NONMETALLIC MINE DETECTION 


27 


must be expected that the mines will not be 
perfectly level in the normal operational plant¬ 
ing of mines. 

When the device had been dismantled, trans¬ 
ported to the vicinity of Sea Cliff, Long Island, 
and reassembled for field tests, it was found 
that there was a severe height effect in which 
false signals were obtained at definite heights 
above the ground. Location was erratic and the 
performance in the field was not considered 
satisfactory. A further test on the instrument 
after it was returned to the laboratory revealed 
that it must have been a fortuitous combination 
of adjustments which had made it operate so 


10-cm wavelength, known as the BAD, is still 
under development. It is not anticipated that 
this detector will be superior to the AN/PRS-1 
type as a portable hand-carried unit, but it is 
hoped that it may prove useful for some other 
application, such as a vehicle-mounted detector. 

Some advances have been made in the 300-mc 
range over the AN/PRS-1. Height compensa¬ 
tion has been greatly improved. Circuits and 
mechanical parts have been simplified, resulting 
in a considerable reduction in weight. However, 
very little progress has been made toward re¬ 
liably detecting small anti-personnel mines. In 
addition to fundamental limitations imposed by 



well previously in the laboratory. With the 
termination of the contract it has been impos¬ 
sible to make a detailed analysis of the behavior 
of the device in order to account for this varia¬ 
tion in results. Since the first experiments 
reveal that it is possible to obtain adjustments 
of the equipment which yield very satisfactory 
operation, it would seem advisable that further 
work be carried out on this method to ascertain 
its fundamental limitations and advantages. 

Summary 

Investigation of the frequency range up to 
10,000 me did not result in the development of 
a detector superior in performance to rede¬ 
signed models of the AN/PRS-1 which operate 
at or near 300 me. A detector operating at a 


wavelength considerations on the size of de¬ 
tectable objects, it was apparent that further 
increasing the sensitivity of the AN/PRS-1 to 
yield a better probability of detecting anti¬ 
personnel mines so increased the number of 
false indications that an instrument less useful 
for field work resulted. It was concluded, there¬ 
fore, that a U-H-F detector of the AN/PRS-1 
type was essentially an anti-tank mine detector 
rather than an anti-personnel mine detector. 
Unfortunately, even in detecting the larger 
mines, modified AN/PRS-1 detectors are not 
completely reliable for all conditions. Ex¬ 
tremely dry soil and extremely wet soil cause 
major detection difficulties. False indications 
have been reduced, but they are still annoyingly 
present. 


CONFIDENTIAL 


















































28 


DETECTION OF LAND MINES 


1 ’ 4 ’ 5 Seismic or Electromechanical 
Detectors 

One of the earliest attacks on the problem of 
detecting nonmetallic mines was based on the 
employment of sonic vibrations in the ground. 
The three principal variations in this method 
which were investigated are as follows: 33 ’ 44 
(1) detection based on variations in the me¬ 
chanical transmission characteristics of the 
earth between two points, (2) detection based 
on reflection of pulsed vibrations from buried 
objects to the soil surface, and (3) detection 


ing mechanism. The wave form of pulses 
transmitted through the ground was observed, 
and after several unsuccessful attempts to 
obtain intelligible reflections, work on this 
system was discontinued. 

The method indicated in Item 3 led to the 
design and construction of nonmetallic mine 
locator equipment. Two different laboratories, 
the Sun Oil Company Geophysical Laboratories, 
Beaumont, Texas, and RCA-Victor Division, 
Indianapolis, independently designed instru¬ 
ments based on the mechanical impedance prin¬ 
ciple. Both designs were practically identical. 



based on variations in the mechanical im¬ 
pedance of the earth. 

In investigating the principle outlined in 
Item 1 above, simple measurements were made 
of the transmission of a pulse through soil in 
which a mine was buried, and these results 
were compared with normal soil. The difference 
between the two measurements was found to 
be less than that which could be introduced 
erratically by variation in the driver and pick¬ 
up contact with the ground. 

There were two outstanding difficulties in 
obtaining reflection indications (Item 2 above) : 
the very high rate of absorption by the ground 
of a steep wave front, and the generation of 
spurious waves within the ground by the driv- 


In practice these detectors were able to locate 
both metallic and nonmetallic mines, but cer¬ 
tain limitations in performance (described 
later) prevented them from becoming useful 
field equipment. 

Instrument Description 

These detectors comprised essentially: (1) 
an oscillating mass, capable of being electrically 
driven, held in contact with the ground, (2) 
a vibration pickup device mounted on the oscil¬ 
lator, and (3) an electric network—a feedback 
amplifier connecting oscillator and pickup. 
These elements, together with the ground 
immediately under and surrounding the de¬ 
tector, constitute a resonant system. Whenever 


CONFIDENTIAL 























































































NONMETALLIC MINE DETECTION 


29 


ground conditions are changed by the presence 
of a mine or disturbed earth, the resonant 
frequency is usually low; when the ground is 
normal, the resonant frequency is higher. 

A circuit diagram for one of these detectors 
is shown in Figure 18, and photographs of the 
apparatus are shown in Figures 19 and 20. 
The shift in the resonant frequency charac- 


w 



Figure 19. Prototype instrument. 


teristic of such a system is shown in Figure 21. 
This frequency shift may be observed on a 
simple frequency meter or may be indicated as 
a tone in a headset. 

Performance and Evaluation 

Unfortunately, a point-to-point investigation 
of the ground is necessary with this equipment 
since the range of the seismic detectors is 
limited to approximately 6 in. from the point 
of contact. With this range it has been esti¬ 
mated that a 3-ft strip of ground can be 
checked at the rate of about 15 fpm. It is doubt¬ 
ful that these detectors can compete with prod¬ 
ding from the point of view either of reliability 


or of speed. One further limitation is that 
contact with the ground is necessary. Since the 
transducer heads may weigh 6 lb or more, it 
would be dangerous to operate them in areas 
containing low-pressure-functioning anti-per¬ 
sonnel mines. 

Models of two different types of mechanical 
impedance detectors (25 of each type) were 
procured for field trials by the U. S. Army 
Engineer Board. The above limitations, brought 
out by these field trials, plus the high promise 
shown by the AN/PRS-1 type of detector, 
forced the decision to discontinue work on 
seismic detectors at that time. 


146 Detector Set AN/PRS-5 (XB-2) 

(Beach Detector) 

The development project which resulted in 
the design and construction of the beach de¬ 
tector was initially aimed at exploring a method 
of detecting nonmetallic mines by observing 



Figure 20. Prototype instrument as adapted for 
use when crawling. 


short-distance changes in ground resistivity 
using a contact method, the electrodes being 
metallic wheels pressed hard against the 
ground. Several wheel-electrode arrangements 
were made and circuits suitable for use with 
them devised. It was found that mines could 
be detected in certain types of soil, but that in 
many others detection was difficult or impos¬ 
sible. 3 - 43 Thus, early in the project, attention 


CONFIDENTIAL 



30 


DETECTION OF LAND MINES 


was turned to further investigations of ap¬ 
paratus employing an inductive method in 
which no direct contacts were made with the 
ground, a modification of the mutual inductance 
bridge-type detector. This program resulted 
in the development by the Electro-Mechanical 
Research Company, Houston, Texas, the NDRC 
contractor, of the beach detector, designated 
AN/PRS-5 (XB-2) by the Engineer Board. 

Description of Instrument 

Figure 22 is a photograph of the completed 
detector. 2 Figure 23 is a schematic diagram of 



Figure 21. Curves to show frequency character¬ 
istics of transducer off of and over M-5 mine 
buried 3 inches deep. 


the layout with the coil system shown in more 
detail. Figure 24 is a block diagram of the 
complete circuit, and Figure 25 is a simplified 
circuit diagram for the detector set. 

The outstanding characteristic of this de¬ 
tector is its ability to detect both metallic and 
nonmetallic mines on beaches and in other soils 
where the soil conductivity is extremely high. 
This characteristic is, of course, primarily due 
to the choice of an r-f (465-kc) energizing 
field, which, in terms of the analysis previously 
given, enhances the eddy-current or conduction 
effect in detection. 

The circuit design contains a few novel fea¬ 
tures which are important in the operation of 
the detector. First, a type of phase discrimina¬ 
tion system was introduced to discriminate 
against the inductive reactance signal, for 
reasons which have been mentioned before. 
Phase selection (see Figure 24) is obtained 
essentially by the introduction in the circuit 
of a strong, permanent resistive component 


which combines in the receiving coil with the 
signal from the ground and from a mine. Vector 
combination of complex voltages is used to 
make the detector less sensitive to inductive 
signals; that is, the combined emf from the 
mine signal, ground signal, and permanent 
resistive component is applied to an amplifier 
which automatically reduces the total emf by 
an amount corresponding to the permanent 
component. The output signal, therefore, meas¬ 
ures the changes in magnitude of the impressed 
emf, the signal being larger for a resistive 
change than for an inductive change (out of 
phase) if the two changes are of equal magni¬ 
tude. The final circuit is then a derivative type, 
which makes detection possible only if the de¬ 
tector is in motion above the mine (not if it is 



Figure 22. AN/PRS-5 (XB-2) Beach detector. 


held in a static position). Second, an automatic 
balancing system is utilized which quickly com¬ 
pensates any unbalance appearing in the 
circuit. Thus, sweeping over a mine results in a 
vigorous signal for one direction of sweep, 
whereas there is a double and less vigorous 
signal in the opposite direction. One advantage 


CONFIDENTIAL 
























NONMETALLIC MINE DETECTION 


31 


of the automatic balancing system is to mini¬ 
mize the importance of drift and other factors 
which make frequent rebalancing necessary. 2 * 3 

Performance and Evaluation 

The overall performance of the AN/PRS-5 
(XB-2), or beach detector, compares very 
favorably with the SCR-625. Its main ad¬ 
vantages are: (1) its ability to detect all types 
of mines, both metallic and nonmetallic, in 


HEADPHONES OR RESONATOR 



Figure 23. Schematic diagram of AN/PRS-5 
(XB-2) detector (Type SW-7). 


conductive soils, and (2) its partial discrimina¬ 
tion against unwanted inductive signals from 
magnetic soils. Over ordinary terrain the de¬ 
tector has slightly less sensitivity for metallic 
objects than the SCR-625. 

Three models of this detector were con¬ 
structed and tested at various Army field sta¬ 
tions. These trials indicated that the AN/PRS-5 
(XB-2) might have application as a special 
detector for amphibious operations due to its 
excellent performance on beaches. The Army 
and Marine Corps decided, however, that the 
need for this locator was outweighed in impor¬ 
tance by the undesirability of adding further 
special-purpose equipment—even though it was 
pointed out that the device is a capable metal 


detector at all times. In case a future require¬ 
ment for the beach detector arises, it should 
be noted that its performance can be markedly 
improved, particularly with regard to phase 
discrimination. 


14,7 Earth Current Detector 

The development of a system of mine detec¬ 
tion in which the required electromagnetic 
energization is supplied at a distance from the 
immediate location, the detector itself being 
without an energizing field, was first exten¬ 
sively considered by the British. Their work 
resulted in the development of the British de¬ 
tector set X-7, a unit capable of detecting 
metallic mines reliably. The British lack of suc¬ 
cess with this set in detecting nonmetallic 
mines, together with the cumbersomeness of 
the equipment, at first deterred Section 17.1 
from investigating the possibilities of this 
method further. Two factors were responsible, 
finally, for the decision to attempt a solution 
using this approach: (1) a German intelligence 
document was captured in which was described 
a countermeasure against Allied mine de¬ 
tectors—this countermeasure was a mine 
actuated by the electromagnetic field surround¬ 
ing a detector; (2) the general lack of success 
of other contractors in making significant im¬ 
provements in the U-H-F-type detector or in 
developing new systems with superior per¬ 
formance. 

In the so-called earth current method a large 
loop of cable or, alternatively, an electrode con¬ 
figuration capable of establishing currents 
along the surface of the soil is used. Anomalies 
in the field established by the energy source 
are then detected by a sensitive pickup, such as 
a coil system, at a distance from the source. 
(Countermeasure considerations involved in 
this detection method are discussed in Section 
1.6.2.) Contracts were let in April 1944 to two 
research laboratories, the Shell Oil Company 
and the Elflex Company, 4 both of Houston, 
Texas, for the development of the earth current 
detector. Work on both developments was in 
progress at the close of World War II. Although 
these projects were not finished, the work which 


CONFIDENTIAL 


























32 


DETECTION OF LAND MINES 


was completed permitted the drawing of rather 
important conclusions concerning the possibili¬ 
ties for detection by this general method. 

The sensitivity of the ECD performing most 
satisfactorily at the close of the project may 
be summarized as follows. 42 With an estimated 
exciting field of 100 microgauss in the area 
under test (3 amp flowing in a two-turn loop), 
the depth of detection for an 11-in. diameter, 
% in. thick iron disk is approximately 2 ft; 
for a 31 / 2 -in. OD, 6 in. long piece of iron pipe 
planted with its axis vertical, the depth of 


with audio-frequency methods. They indicate, 
further, that nonmagnetic conductors must ex¬ 
ceed a certain mean horizontal cross section, 
depending on their resistivity and the exciting 
frequency, if their effect is to be comparable 
with that of a ferromagnetic object of the same 
size and shape. It is possible that much higher 
frequencies will yield more satisfactory results 
in the detection of nonmetallic mines. 

Although to a certain extent specialized 
equipment, the ECD development appears to 
be worthy of further investigation. It is quite 



Figure 24. Block diagram of complete circuit. 


detection is 1T/2 ft; for a 314 -in. OD brass tube 
of Vs-in. wall, 8 in. long, it is only 6 in. A 
6-32 iron screw, % in. long, planted vertically 
flush with the surface, is just detectable. A 
similar brass screw has no effect. The a-c 
frequency of the power source during these 
observations was 1,000 c. Figure 26 is a photo¬ 
graph of the pickup which yielded the above 
results. It is believed that the general design 
of the pickup itself is very nearly satisfactory 
for the frequencies involved. Compensation or 
balancing of this pickup is believed to be suffi¬ 
ciently good to allow approximately a tenfold 
increase in exciting field strength. Thus with 
the above sensitivities, detection at consider¬ 
ably greater depths of burial can be achieved. 

Theoretical considerations indicate that little 
hope is justified for the detection of insulators 


possible, particularly in view of British ex¬ 
perience, that the detector would be useful 
simply as a metal detector. Its countermeasure 
aspects further support the desirability of ex¬ 
ploiting the method. It also seems reasonable 
to investigate the possibilities of the employ¬ 
ment of a higher frequency for locating non¬ 
metallic as well as metallic mines. 


148 Radioactive Detectors and the Marking 
of Friendly Mines 

Introduction 

This section is concerned with two quite 
separate problems linked together by the use 
of a common instrument—a y-ray detector or 
Geiger-Mueller counter. Project Mamie was the 


CONFIDENTIAL 
















































NONMETALLIC MINE DETECTION 


33 


extension of the general principle of radioactive 
tracers to the marking and subsequent identi¬ 
fication and location of military materials, espe¬ 
cially booby-traps, anti-tank mines, and lanes 
through minefields. Its most practical applica¬ 
tion in World War II (albeit German) was to 
the marking of nonmetallic, anti-tank mines 
so that friendly troops would be able to relocate 
them easily. Project Dinah, as the whole non¬ 
metallic mine-detector program was known, 


The Mamie problem, which involves the de¬ 
velopment of a safe, inexpensive, radioactive 
substance to serve as markers, and a rugged 
but very sensitive y-ray detecting device, was 
solved adequately by the use of activated cobalt- 
60 in the form of small buttons, and through 
the use of a modified Geiger-Mueller counter. 
The conclusion of the Dinah investigation was 
that nonradioactive, nonmetallic, anti-tank 
mines, when buried 3 in. under ground, cannot 



included an extension of the principles of radio¬ 
activity to the detection of enemy nonmetallic 
mines. In this, three general methods were 
investigated: (1) detection due to the shielding 
effect of a mine on the natural radioactivity 
of the ground, (2) variations in the scattering 
of y-rays from the ground, and (3) various 
effects associated with the bombardment of the 
ground by neutrons. 

Detection in this last case might be based 
on: (1) measurement of direct neutron scatter¬ 
ing, (2) detection of y-ray emission occurring 
instantaneously during bombardment due to 
neutron reactions, or (3) measurement of y-ray 
emission after neutron bombardment has 
ceased, this emission being due to the produc¬ 
tion of artificial radioactive elements. 


be detected at practical scanning speeds by any 
of the methods attempted. 

Mamie or AN/PRS-2 

The most sensitive of the y-ray detectors was 
standardized and placed in limited procurement 
by the Office of the Chief Signal Officer. The 
detector, known as the AN/PRS-2 by the 
Engineer Board, is shown in Figure 27. It was 
developed by the Texas Company’s Geophysical 
Laboratory at Houston, Texas. 46 It consists of 
three parts: a head (which is the y-ray detector 
proper), a box (which contains the power 
supply and amplifier), and a set of earphones. 
For the y-ray detector a special instrument is 
used, the principle of which was not disclosed 
by the Texas Company. (It had been developed 


CONFIDENTIAL 




























































































34 


DETECTION OF LAND MINES 


prior to the NDRC contract.) It is known that 
the detector operates along the lines of a 
Geiger-Mueller counter, but it has a much 
higher efficiency than an ordinary counter for 
y-rays. The detector is of all-metal construction 
and is cylindrical in shape, with a diameter 
of 2 in. and a length of 4 in. Current pulses 
from the detector are amplified and equalized 
with regard to their amplitude and width. The 
frequency of the pulse is an indication of the 
intensity of the y-ray field. Instead of measur- 



Figure 26. ECD pickup in operation. 


ing the frequency, the pulses are fed into an 
integrator, the voltage output of which is pro¬ 
portional to the pulse frequency and, therefore, 
to the y-ray intensity. The magnitude of the 
voltage is indicated by an audible tone, its 
intensity increasing with increasing y-ray in¬ 
tensity. A circuit diagram for the instrument 
is shown in Figure 28. 

The sensitivity of the detector is such that 
it can very easily detect 2 microgram (/xg) 
equivalents of radium buried 2 or 3 in. in the 
ground. It is interesting to note that this sensi¬ 
tivity is approximately the same as that of the 
German detector, Stuttgart 43, designed for 
the same purpose, i.e., marking of their Topf 
mine. 

The radioactive markers were designed and 
produced by another NDRC contractor, the 


Radiation Center, Department of Physics, 
Massachusetts Institute of Technology, 31 which 
also investigated the Mamie and Dinah prob¬ 
lems from the radioactivity viewpoint. The 
markers were supplied in one case as a solution 
of cobalt-60 chloride in small glass ampoules 
containing 2.5 microcuries (/xc), which could be 
buried intact near or over a friendly mine. 
According to the work of this contractor, 1 
n c of cobalt-60 equals 2.0 /xg “radium y-ray 
equivalent units.” The arbitrary definition of 
“radium y-ray equivalent units” in this case 
is taken to be the relative y-ray intensity of the 
cobalt compared with the y-rays of 1 /xg of 
radium element in equilibrium with its decay 
products, when both y-rays are filtered through 
1.1 cm of lead and measured on a platinum- 
screen-cathode counter. 

There are, of course, a number of variations 
in the way a radioactive cobalt could be sup¬ 
plied as a marker. Toward the close of the 
program, markers were supplied in the form 
of artificial rocks containing activated cobalt. 
These rocks were spheres of a porous ceramic 
material saturated with an aqueous solution of 
cobalt nitrate which was subsequently con¬ 
verted to the oxide by ignition (so that the 
cobalt would be insoluble). As described in the 
literature, 48 - 49 activated cobalt does not present 
the health hazard of radium because (1) it is 
not biochemically similar to calcium as radium 
is, and (2) it emits no a-rays (which subse¬ 
quently destroy the bone-regenerating cells). 

Dinah 

The following is a summary of the conclu¬ 
sions drawn from the investigation of radio¬ 
active methods of detecting enemy mines. It 
was on the basis of these conclusions that the 
entire method was discarded. 

Shielding Effect Principle. The application 
of this principle is impractical for two reasons: 
(1) the observation time d for a measurement is 
prohibitively long; (2) the enemy mine, as is 
the case with some American M-5 mines, may 

d It will be remembered here that the accuracy of 
counting for all counter tubes is proportional to the 
counting time. Thus, in attempting to distinguish be¬ 
tween two counting rates which are slightly different, 
the time of counting, since it is a statistical problem, 
enters as an important factor. 


CONFIDENTIAL 







NONMETALLIC MINE DETECTION 


35 


carry an amount of radioactive contamination 
which is too large to present much of a contrast 
to the surrounding soil. 46 

Scattering of y-1Rays. The method is applica¬ 
ble only in a few restricted cases. 

Scattering of Neutrons. The penetrative 
power is limited. Measurements are affected by 
the distance between the instrument and the 
ground, and they depend very much on the 
water content of the ground. 

y -Rays without Time Delay from Neutron 
Bombardment. The observed effects have noth- 


Figure 27. AN/PRS-2 detector (Mamie). 

ing to do with neutrons, and they can be 
ascribed to the variation in the geometrical 
arrangement of the measuring instruments. 

Delayed y -Rays from Neutron Bombardment. 
The effect is only of the order of the variations 
one finds over different parts of undisturbed 
ground. The measuring time is too long. 

1,4 9 Evaluation of Nonmetallic Mine- 
Detection Program 

The development of a locator of nonmetallic 
mines which will meet military field require¬ 
ments is admittedly a difficult problem. The 
physical contrast between a mine and the soil 


in which it is buried may be small, and the 
difference between a rock and a buried mine 
usually is indistinguishable. A number of par¬ 
tial solutions resulted from the NDRC pro¬ 
gram. The most satisfactory of these was the 
AN/PRS-1 type, which was procured and de¬ 
livered to the theaters of operation. This de¬ 
tector does not meet all military requirements, 
mainly because it is not 100 per cent reliable 
in the detection of both anti-personnel and 
anti-tank nonmetallic mines. The seismic detec¬ 
tor is capable of locating both metallic and 
nonmetallic mines at shallow depths of burial. 
It does not constitute a satisfactory detector 
because of its limited area of coverage and 
because contact with the ground is necessary. 
The beach detector has only very limited appli¬ 
cation for nonmetallic mines (namely, when 
planted in highly conducting soil), although it 
is a good metal detector. 

That a completely adequate solution was not 
achieved, it is believed, is due to the inherent 
difficulty of the problem rather than any lack 
of resourcefulness or ingenuity on the part of 
NDRC, the Engineer Board, and their various 
contractors. Indeed, as compared with the prog¬ 
ress made by other nations, the U. S. program 
resulted in by far the most satisfactory solu¬ 
tion, and this solution was available for field 
use in time to affect the course of World War II, 
should it have been needed. 

The development program carried on by 
NDRC was not one of great magnitude. At 
various times nine different contractors, repre¬ 
senting geophysical development laboratories, 
research laboratories of the electronics indus¬ 
try, and laboratories of educational institutions, 
investigated means of solving the problem. 
The total NDRC expenditure on these contracts, 
including the procurement of small numbers 
of pilot models, was somewhat less than 
$500,000. 

It is possible that more significant results 
might have been obtained if NDRC had pur¬ 
sued the policy of concentrating the work in 
one large laboratory, rather than using a di¬ 
versified series of contractors. In August 1945 
plans were under way to attempt, even at that 
late date, a consolidation of effort in a single 
NDRC laboratory at the Engineer Board, Fort 




CONFIDENTIAL 




36 


DETECTION OF LAND MINES 


Belvoir, Va. In retrospect, it is difficult to see 
how such a laboratory in this case would have 
significantly affected the final results, although 
there appeared to be general agreement among 


fundamental changes or advances in the tech¬ 
nique of detection of nonmetallic mines are now 
in prospect on the basis of methods tried during 
World War II and described in this report. As 



representatives of NDRC and the Army that 
such a laboratory, staffed with first-class per¬ 
sonnel, would have expedited the program at 
that stage. 

It seems clear at the present writing that no 


the problem is a continuing one which undoubt¬ 
edly will have to be met in any future war, it 
is apparent that there is an urgent need for new 
approaches and new ideas. 

Perhaps the most adverse criticism, if one 


CONFIDENTIAL 






















































































































NONMETALLIC MINE DETECTION 


37 


need be made, of the nonmetallic detector de¬ 
velopment program is one of its timing. Not 
long after the AN/PRS-1 was in production it 
became apparent that this detector would not 
adequately fulfill all the military requirements, 


and result in a more useful field detector. How¬ 
ever, a year after these models were first made, 
the design of combination detectors was in the 
same state of development because no further 
work was done with them. The failure to pur- 



Figure 29. AN/PRS-1 (XB-2) detector—set components. 


and it was shortly thereafter that combination 
models of the AN/PRS-1 with available metal 
detectors were constructed by various NDRC 
contractors. These models, as will be described 
in the next section, apparently eliminate some 
of the limitations inherent in the AN/PRS-1 


sue this promising engineering advancement 
was probably due to the fact that NDRC and 
the Engineer Board were counting heavily on 
improvements in the design of the AN/PRS-1 
type of detector to meet field objections. Toward 
the close of the program every effort was made 


CONFIDENTIAL 

















38 


DETECTION OF LAND MINES 


to expedite the design and construction of these 
units and to make up for the time lost. 

15 UNIVERSAL MINE DETECTION 

1,51 Introduction 

The ultimate objective of a mine locator re¬ 
search program is the development of a detector 
that will locate under field conditions all types 



Figure 30. AN/PRS-1 (XB-4) detector—set 
components. 


of mines in use by the opposing forces. Such 
a device, which may be termed a universal mine 
detector, has been the goal of the research 
program of the Engineer Board and Section 
17.1, NDRC. The closest approach to a universal 
detector developed by NDRC is the combination 
detector described in this chapter. This detector 
is simply an amalgamation of a U-H-F detector 
and a metal detector. The U-H-F part of the 
combination locates nonmetallic mines (with 
the same fundamental limitations already de¬ 
scribed), and the metal-detecting portion lo¬ 
cates metallic mines and mines which, though 
perhaps mainly of nonmetallic construction, do 
contain some metallic parts. 

Military Requirements 

The basic military characteristics for a com¬ 
bination detector were established by the Engi¬ 
neer Board and the Commanding General, ASF, 


in March 1945. These specifications include the 
usual requirements on weight, portability, rug¬ 
gedness, and watertightness that are standard 
for all Army mine detectors. The performance 
desired was equal to or better than that obtain¬ 
able against a particular enemy mine by the 
best of the elements used singly. In other words, 
no loss in performance due to the combination 
was considered permissible. Another feature 
was that provision be made for a switch per¬ 
mitting the two sensitive elements to be oper¬ 
ated either simultaneously or individually. 


i.s . 3 Description of Apparatus 

The development described in this section 
was not finished at the cessation of hostilities, 
with the result that the models described here 
were rather hastily completed. The principal 
difference between these and true final models 
is that the circuit for the metal-detector ele¬ 
ment does not include phase discriminating 
features. Thus the circuit as shown would give 
false indications for permeable rocks and soil. 

Nonmetal-Detecting Portion 
Of AN/PRS-6 (XB-6) 

Two combination detectors were under de¬ 
velopment by the same NDRC contractor, the 
RCA-Victor Division, Indianapolis, Ind. 36 One, 
the AN/PRS-6 (XB-4), is a combination of the 
U-H-F detector known as the AN/PRS-1 
(XB-2) with a coil system sensitive to metals; 
and the other, the AN/PRS-6 (XB-6), is a 
combination of another U-H-F detector, the 
AN/PRS-1 (XB-4), with a metal-detecting coil 
array. The two U-H-F detectors (the XB-2 and 
XB-4) are quite different, but the metal detec¬ 
tors are similar. The XB-2 is more directly 
an outgrowth of improvements in the original 
AN/PRS-1. A photograph of this detector set 
is shown in Figure 29. An important feature 
of this detector is the placing of the r-f oscil¬ 
lator on the detector head, as in the AN/PRS-1. 
The XB-4 (shown in Figure 30) utilizes quite 
a different antenna and receiving array; in 
addition, it is a so-called transmission-line 
model with the U-H-F oscillator in the small 
amplifier pack. A circuit diagram for this in- 


CONFIDENTIAL 







UNIVERSAL MINE DETECTION 


39 


strument is given in Figure 31. It is this latter 
model, the AN/PRS-1 (XB-4), whose modifica¬ 
tion to a combination detector AN/PRS-6 will 
be described. 

The nonmetal-detecting portion of the 
AN/PRS-6 is a balanced radiation type of 
detector. The head of this detector model (see 
Figure 30) includes three antennas, one for 
receiving and two for transmitting. The phys¬ 
ical size of these antennas has been reduced 



Figure 31. AN/PRS-1 (XB-4) detector—sche¬ 
matic diagram. 


to a minimum by using both inductive and 
capacitive loading. The function of the capacity 
plates is to compensate for variations in the 
height above ground at which the detector 
might be operated, thus reducing the sensitivity 
of the detector to tilt. It was found that tilt 
sensitivity could be reduced further by putting 
the antennas in a different plane, the transmit¬ 
ting antennas being mounted against the inside 
of the top and the receiving antenna near the 
inside of the bottom of the plastic case sur¬ 
rounding the head. 

The outer dipoles (transmitting antennas) 
are loosely coupled through a Twinax trans¬ 
mission line to a 285-mc oscillator, and the 
phase of the energy radiated by one is displaced 
with respect to the other. This displacement 
is such that it causes a null in the field strength 
to exist at the position of the center (receiving) 
antenna. When a mine enters the field of either 
transmitting antenna, an unbalance occurs at 
the receiving antenna which is detected by a 
1N21B crystal. 

As the crystal detector is a device of rela¬ 
tively low impedance (about 50 ohms), a trans¬ 


former is necessary to take full advantage of 
the output of the crystal. This, however, caused 
other serious practical difficulties because a 
mine signal is of very low frequency (approxi¬ 
mately 3 c). The problem was solved by causing 
the U-H-F oscillator to block at a frequency of 
about 1,000 c, resulting in 100 per cent modula¬ 
tion of the radiated wave. The modulated 
285-mc signal is picked up by the receiving 
antenna and demodulated by the crystal detec¬ 
tor. The resulting audio signal is fed to the 
1L4 amplifier over the Twinax cable, which 
also is used to transmit the U-H-F signal to 
the radiators. Isolation of the audio signal from 
the U-H-F signal is effected by the use of r-f 
chokes and a by-pass capacitor. Both sides of 
the Twinax line are connected to the receiving 
antenna through r-f chokes. This arrangement 
prevents U-H-F leakage from unbalancing the 
receiving antenna system, since the leakage 
currents through the two chokes are about 180 
degrees out of phase. In the oscillator the center 
point of the coupling loop is effectively grounded 
at 285 me by a 33-^f capacitor, allowing the 
audio signal to reach the input transformer free 
from radio frequencies. 

The balanced antenna system utilized in this 
detector results in two tones being heard when 
the detector is swept over a mine, one tone 
coming from each side of the mine as the de¬ 
tector passes over it. When the head is held in 
the null position between the two tones, it is 
directly over the mine. 

Improvements in Nonmetal-Detecting Por¬ 
tion over AN/PRS-1. The transmission-line 
unit described above is much superior to the 
AN/PRS-1 detector in a number of important 
features: (1) it is insensitive to changes in 
height above ground, (2) it searches a path 
about 50 per cent wider than the AN/PRS-1, 
(3) it gives a more accurate indication of the 
location of the center of the mine, (4) the 
weight of the instrument is greatly reduced, 
and (5) less skill is necessary in operation. 

The factors involved in weight reduction 
were numerous. An important one was the 
replacement of the type 955 acorn triode of the 
AN/PRS-1, a tube which requires a 900-mw 
filament supply, by a 958-A acorn tube, which 
requires only 125 mw. The use of this tube re- 


CONFIDENTIAL 
































40 


DETECTION OF LAND MINES 


suited in further circuit changes; for example, 
it was found to be impractical to use grid-cur¬ 
rent variation as an indication of an unbalance 
of the circuit, because the grid-current varia¬ 
tion in the 958-A is inherently greater than 
in the 955. The substitution of plate-current 
variation was found to be satisfactory. With 
reduced battery drain, much smaller batteries 
could be employed throughout the detector. The 
above changes resulted in a total weight for 
the AN/PRS-6 nonmetal-detecting circuit alone 


changed. A push-pull oscillator (see Figure 33), 
comprising two 1L4 tubes, energizes the large 
central field coil. The signal picked up by the 
two receiving coils is fed through a transformer 
to a two-stage high-gain amplifier with trans¬ 
former coupling from the last stage to the 
headset. The U-H-F signal is fed into the screen 
grid of the amplifier’s first stage. 

It should be emphasized that the circuit 
shown in Figure 33 is not in final form, because 
of the termination of the research program at 




U-H-F CABLE 


METAL DETECTOR 
BALANCING CONTROLS 


TRANSMITTING 


METAL 


DETECTOR 


CABLE 


TELESCOPING 

HANDLE 


TRANSMITTING 

ANTENNAS 


RECEIVING 

COILS 


RECEIVING 

ANTENNA 


Figure 32. AN/PRS-6 (XB-6) detector—set components. 


of less than 5 lb. The detector head itself 
weighed only about 2 lb. 

The Combination Detector 

The combination detector AN/PRS-6 (XB-6) 
in its final development stage by NDRC is pic¬ 
tured in Figure 32, which shows the mounting 
of the coils in the U-H-F head. A schematic 
circuit diagram for this detector set is given 
in Figure 33. Figure 34 is a photograph of the 
amplifier and control pack assembly. 

The metal-detecting part of the AN/PRS-6 
(XB-6) is quite similar to the SCR-625 in cir¬ 
cuit detail, although the coil configuration is 


the close of World War II. If the model had 
been carried to completion, the metal-detecting 
circuit would have been augmented by some 
type of phase discriminating feature, enabling 
it to reject unwanted signals from permeable 
ground and other effects previously described. 


15 4 Evaluation of Combination Detector 
and Universal Detection Program 

The utility of the model herein described is 
that it demonstrates that a metallic search-coil 
system can be placed in close proximity to the 


CONFIDENTIAL 





UNIVERSAL MINE DETECTION 


41 


U-H-F antenna system without reducing ap¬ 
preciably the sensitivity of either component. 
With the arrangement in the AN/PRS-6 
(XB-6), a sensitivity to metals meeting the 
Engineer Board specification has been ob¬ 
tained. 36 Although forthcoming field trials may 
reveal that the combination causes reduced sen¬ 
sitivity in the nonmetal-detecting portion, this 
reduction is not expected to be serious. 


phases of the universal detection problem which 
still remain unsolved: (1) reliable detection 
of small anti-personnel nonmetallic mines or 
small mines containing insufficient metal to be 
detectable by the metal-locating part of the 
combination, and (2) certain inherent unrelia¬ 
bilities in the performance of the nonmetal¬ 
detecting portion. From previous experience it 
appears certain that the above two objections 



It is interesting to note that the combination 
detector is lighter and less bulky than the orig¬ 
inal model of the AN/PRS-1. At the same time 
performance has been improved and operation 
simplified. The addition of the phase discrim¬ 
inating feature in the circuit will increase its 
weight somewhat, but it seems improbable that 
this will have unfavorable practical conse¬ 
quences. 

The combination detector AN/PRS-6 (XB-6) 
appears to satisfy the requirements for a uni¬ 
versal detector. There are, however, certain 


will be present in the AN/PRS-6; it is quite 
possible that field tests of this combination lo¬ 
cator will bring out other objections which may 
be serious. Until these trials are made, it is 
difficult to evaluate its performance adequately. 

Once it was decided to construct a combina¬ 
tion detector from the ground up, the experi¬ 
mental program progressed rapidly and very 
satisfactorily. It is felt that the work was han¬ 
dled competently and efficiently by the NDRC 
contractor. 

In order to obtain a true universal detector 


CONFIDENTIAL 















































































































































42 


DETECTION OF LAND MINES 


(one which will detect all mines—large and 
small, nonmetallic and metallic—in a wide va¬ 
riety of soil conditions), it is apparent that 
considerable further research work will have 
to be undertaken. One possibility may be to 
utilize some variation of a facsimile scheme 
which will enable the operator to discriminate 
against stones, roots, and other objects which 


16 COUNTERMEASURES, ANTI-COUNTER- 
MEASURES, AND A CONTINUING 
RESEARCH PROGRAM 

1,6,1 Introduction 

Land-mine warfare is typical of most combat 
techniques in that it is not static. Its evolution, 


Figure 34. AN/PRS-6 (XB-6) amplifier with case removed (U-H-F oscillator section). 



B BATTERY 


METAL DETECTOR 
VOLUME CONTROL 


U-H-F DETECTOR 
VOLUME CONTROL 


1L4 FIRST AMPLIFIER 

JT TRANSFORMER 
PLATE REACTOR 

TRIMMER CAPACITOR 

iL 4 SECOND AMPLIFIER 
FILTER CHOKE 

OUTPUT TRANSFORMER 

OSCILLATOR TRANSFORMER 
1 L 4 OSCILLATOR TUBES 

U-H-F OSCILLATOR 


FEED-THROUGH CAPACITORS 


give indications (at least in U-H-F detectors) 
identical to buried mines. The Engineer Board 
has recently (fall 1945) let a contract with a 
development agency to investigate this method 
in a long-range program. It is to be hoped that 
from time to time other ideas will arise which 
may become the basis for the design of new 
detectors, and that facilities will be available to 
reduce these ideas to practice. 


however, has been at a slower rate during World 
War II than many other forms of combat in 
the sense that specific countermeasure devices, 
and therefore instruments designed to counter 
the countermeasures, were never widely used. 
Countermeasures for standard army mine de¬ 
tectors were developed both in this country 
and by the Germans (and perhaps by other 
countries), but no reported instance is known 


CONFIDENTIAL 




COUNTERMEASURES, ANTI-COUNTERMEASURES 


43 


of their use against the U. S. Army. Intelligence 
reports indicate restricted employment of a 
mine-detector countermeasure against the Rus¬ 
sian army and the intended use of this same 
device against the Allies in the European the¬ 
ater. There has been no report of such an in¬ 
strument development by Japan or other enemy 
countries. 

Perhaps the major reason for the lack of 
employment of a countermeasure against the 
mine detector was that no new development, 
either in the form of specific instruments or 
new tactics, seriously weakened the strength of 
the land mine as a defensive weapon. A mine¬ 
field containing a high density (say four or five 
mines per yard of front) of both anti-tank and 
anti-personnel nonmetallic and metallic mines 
and covered by supporting fire, both anti-per¬ 
sonnel and larger caliber, could not be breached 
by frontal assault without sizable losses in both 
men and vehicles at the close of World War II. e 

It is apparent from the discussion in this 
report that no detector was capable of locating 
all known enemy mines reliably. Hand probing 
with some type of exploring rod comes perhaps 
the nearest to a solution from the detection 
viewpoint. This procedure, however, is slow, 
dangerous, and difficult, and not sufficiently 
effective to necessitate the employment by the 
enemy of extensive countermeasures; it is at 
best an expedient and does not lend itself to 
rapid break-through of mined positions. 

It can be argued, however, that the introduc¬ 
tion of mine-detection countermeasures would 
have increased greatly the difficulty of penetrat¬ 
ing minefields, and it can be reasonably assumed 
that countermeasures and anti-countermeasures 
will play an important part in the mine warfare 
of the future. 


1 ' 6 ' 2 Countermeasures and Anti- 
Countermeasures 

The German countermeasure against mine 
detectors was a simple device. It consisted of 

e For example, see report of Engineer Officer, First 
Infantry Division, on the employment by that division 
of land mines in the Battle of the Bulge. 


a mine with an influence fuze sensitive to the 
field put out by the mine detector. The counter¬ 
measure in this case comprised a few turns of 
wire forming a partially tuned circuit and a 
sensitive relay (operating on about 20 /xa) 
which was actuated by the induced current in 
the coil. The tuning in this particular device was 
sufficiently narrow so that it could be directed 
only at a single type of enemy mine detector. 
It could not be operated, for example, both by 
a U. S. detector (which works at 1,000 c) and 
by a Russian detector (which uses a frequency 
of about 3,000 c). 

The effectiveness of this countermeasure can 
be decreased by reducing the field strength 
developed by the field coil of the detector. This 
practice, however, decreases the sensitivity of 
detection. A more adequate anti-countermeas¬ 
ure has been developed by a contractor of the 
Engineer Board. In this device the field strength 
of the detector is automatically reduced by the 
mine signal picked up in the detector. With this 
circuit it has been possible to detect safely 
metallic mines fuzed with circuits similar to 
but more sensitive than the German counter¬ 
measure. The overall mine-detection sensitivity 
of this circuit approaches very nearly that of 
the best Allied mine detectors. 

Undoubtedly, the next step would be to make 
an influence fuze for a mine which is consider¬ 
ably more sensitive; in a race of this type it is to 
be expected that the countermeasure (i.e., the 
mine) will ultimately defeat the detector, since 
the detector is operating against an inverse- 
sixth-power law while the fuze (in making use 
of the field of the detector directly) is function¬ 
ing on an inverse-cube law. The disadvantage 
of this scheme from the countermeasure point 
of view is the need for designing and construct¬ 
ing rather elaborate and delicate fuze mech¬ 
anisms. 

Countermeasures of the above type may be 
set off or exploded harmlessly by the employ¬ 
ment of equipment similar to the ECD. The 
field established by the energizing system of 
this instrument could be used to trip the coun¬ 
termeasure before sweeping is undertaken with 
a standard detector; or alternatively, sweeping 
could be undertaken more safely with the ECD 
itself. Equipment designed to detonate re- 


CONFIDENTIAL 




44 


DETECTION OF LAND MINES 


motely booby-traps of the type under considera¬ 
tion could undoubtedly be made to permit 
sweeping through a rather wide frequency 
range. 

Equipment of the type described above, using 
a different frequency range, could be made 
effective against another type of electrical mine 
fuze. Section T, NDRC, developed during the 
war a so-called proximity fuze which operated 
when it approached another object. It was used 
in an influence mine which is a mine-detector 
countermeasure. This mine is sensitive to the 
presence of a foreign body near the fuze; hence, 
it would be a countermeasure against any mine 
detector, irrespective of whether an exploring 
electromagnetic field is associated with it or 
not. 

A third device which may be considered a 
countermeasure is a mine fitted with a mechan¬ 
ical tilt igniter. Such an igniter was developed 
by the Germans during World War II. The mine 
is exploded when the mine detector head sweeps 
against the rod part of the tilt igniter, which 
extends 6 in. or more above the surface of the 
ground. This device would probably be most 
effective at night, when it would be impossible 
to see a well-camouflaged tilt rod. The wide- 
scale employment of this as a countermeasure 
would necessitate, if nothing else, a rather com¬ 
plete change in standard operating procedures 
with mine detectors; conceivably, it might also 
demand certain design changes in mine detec¬ 
tors. The device would be particularly trouble¬ 
some if it could be made of completely non- 
metallic materials. 

Small trip wires connected between mines 
may also be considered as a countermeasure 
against mine detectors. To overcome this ob¬ 
stacle, a small trip-wire detector, which can be 
attached to a regular detector, has been devel¬ 
oped, but not placed in use. It is anticipated 
that the trip-wire detector would be fairly sat¬ 
isfactory as an anti-countermeasure. 

From the general countermeasure point of 
view, one of the most satisfactory devices would 
be simply the development of low-pressure¬ 
functioning mines which could not be detected 
by enemy mine detectors. Small, nonmetallic 
anti-personnel mines are now in this category. 


1,6,3 A Continuing Research Program 

The failure to prosecute a peacetime program 
for the development of new mines, mine detec¬ 
tors, and countermeasures can result in serious 
consequences. This statement is based on the 
assumption that it will be necessary in the 
future to occupy physically territories held by 
an enemy. In such an operation it is reasonable 
to expect that mines and booby-traps will be 
encountered, perhaps in very large numbers. 
Under these circumstances the possibility of a 
single enemy countermeasure rendering all U. S. 
mine detectors useless can best be guarded 
against by a thorough and efficient mine war¬ 
fare research and development program. 

The objectives of such a program can be 
stated clearly. From the long-range point of 
view the purpose is the development of a de¬ 
tector capable of locating with 100 per cent 
reliability all types of land mines and booby- 
traps. This detector should be, so far as possi¬ 
ble, impervious to countermeasure technique. 
To anticipate possible countermeasures by the 
enemy, and to permit the use of mines in offense 
by friendly forces, work should be continued 
actively on the development of countermeasures 
to mine detectors (as well as to other methods 
of clearing minefields). This program, which 
is still not well established, might include not 
only countermeasures in the normal sense of 
an influence-type electric fuze, but also the 
development of mines which cannot be detected 
by standard mine-detector practice. 

From a more immediate viewpoint, research 
should be continued to develop a locator of small 
nonmetallic anti-personnel mines and nonme¬ 
tallic anti-tank mines which will be reliable for 
all soil conditions. It appears that the successful 
conclusion of this project will not come from 
simple engineering advances in mine-detection 
techniques now being investigated; it is be¬ 
lieved that a satisfactory solution to this prob¬ 
lem will come only from the application of some 
new and perhaps as yet unknown technique. 
This is one way of stating that considerable 
fundamental research must be undertaken to 
solve this problem adequately. 

Again, from the short-range point of view, 


CONFIDENTIAL 



COUNTERMEASURES, ANTI-COUNTERMEASURES 


45 


the development of detectors which are not 
particularly susceptible to countermeasure tech¬ 
niques is in its infancy. It is to be expected that 
continuing engineering advances, both in the 
development of countermeasures and of non- 
countermeasurable detectors, will result in im¬ 


portant practical advances. In other words, 
there does not exist the same preponderant 
need for fundamental research in these fields 
that there is in the detector field, as research 
of a less fundamental nature can still give very 
useful results. 


CONFIDENTIAL 



Chapter 2 

MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 

By John S. Hornheck 


21 INTRODUCTION 

T he purpose of this report is to summarize 
the technical program prosecuted by Sec¬ 
tion 17.1, Instruments, on mechanical and de¬ 
molition methods of clearing land minefields. 

The report is organized into three divisions: 
a summary, mechanical clearance, and demoli¬ 
tion clearance. No attempt is made to report in 
detail the theoretical and experimental investi¬ 
gations undertaken; these may be found else¬ 
where. Rather it is intended to present sufficient 
material to indicate the scope of the work 
undertaken and the useful results accruing 
therefrom. Attention is directed to the incom¬ 
plete solution as yet obtained to the land-mine 
problem. 

The Service control numbers or projects 
under which the work herein reported was con¬ 
ducted were CE-32 and OD-133. The former was 
a general request from the Corps of Engineers 
for assistance in all phases of land-mine clear¬ 
ance, and the latter was restricted to mechan¬ 
ical clearing methods. 


22 SUMMARY 

The advent of the land mine as an effective 
combat weapon in World War II was responsi¬ 
ble for an intensive research and development 
program to discover means of counteracting 
it. The first and perhaps easiest step in this 
direction was the design and production of 
metallic mine detectors. Mechanical and demoli¬ 
tion methods of clearing minefields were also 
investigated at an early date, but it proved to 
be more difficult to find in these fields devices 
which were as satisfactory as a simple metallic 
mine locator. 

From a tactical point of view the appeal of 
mechanical and demolition methods is the speed 
with which a lane through a minefield can be 
cleared. In detector operations many hours 
must be spent, frequently at night, locating 
mines and removing them or detonating them 


harmlessly in place. Often these operations give 
advance notice to the enemy of an imminent 
attack. If explosives could be placed on a mine¬ 
field in a time interval of a few minutes at most, 
breaching of the field could be achieved quickly, 
permitting motorized troops an element of sur¬ 
prise in an attack. Thus, given the method of 
accurately placing a sufficient explosive charge, 
the armor commander has a powerful weapon 
at his disposal. Similarly, if enemy fire over a 
minefield was neutralized, a suitably designed 
mechanical mine exploder could precede tracked 
vehicles, clearing a path for them as they ad¬ 
vance. Mechanical mine exploders would also 
be quite useful for keeping roadways open and 
clearing rear areas where for some reason it 
was undesirable to attempt to locate the mines 
individually. 


2,2,1 Military Requirements 

American military requirements for mechan¬ 
ical mine exploders are in general quite similar 
to requirements for all combat weapons. The 
device must be capable of doing its assigned 
task efficiently. It must be rugged and easy to 
transport, either under its own power or by 
other means. More specifically, Army Ord¬ 
nance desired a mechanical mine exploder which 
would detonate commonly used enemy mines 
at operational depths of burial (say up to 6 in.). 
Preferably the exploder, if mounted on a tank, 
should not require an auxiliary power source, 
and the exploder plus tank must be easily ma¬ 
neuverable under combat conditions. These 
would include unloading the vehicle from am¬ 
phibious landing craft and the negotiation of 
extremely muddy roads and land areas. Army 
Ordnance also emphasized the desirability of 
obtaining an indestructible mechanical clear¬ 
ing device. (This was usually in conflict with 
the maneuverability requirement.) Sufficient 
ground coverage is necessary so that the ex¬ 
ploder does not occasionally miss mines. It is 
also desirable for the mine exploder to be capa- 


46 


CONFIDENTIAL 


SUMMARY 


47 


ble of returning enemy fire while in the process 
of carrying out its assignment. 

With regard to explosive clearance, Army 
Ground Forces required a safe, accurate means 
of placing an explosive charge, the equipment 
associated with the device to be packaged in a 
form readily transportable, the total weight of 
the device to be as low as possible, and its oper¬ 
ation to be practically foolproof. Time of as¬ 
sembly behind the front lines, if necessary, is 
also an important consideration, as is the time 
required for the device to be placed in the de¬ 
sired position on the minefield. 


Scope of Work 

In the mechanical clearing domain two proj¬ 
ects were undertaken by NDRC. One dealt with 
the development of a heavy, self-propelled, re¬ 
motely controlled roller device, which could be 
operated from a tank or other suitable station 
and maneuvered through a minefield. The sec¬ 
ond comprised design studies of a flail-type 
mine exploder known as the Rotaflail and the 
building of models. This latter project was the 
only one of the two which appears to be par¬ 
ticularly promising and therefore attention is 
restricted to it. NDRC development work on 
the Rotaflail comprised: (1) development of 
the Rotaflail principle and a mathematical anal¬ 
ysis of it, a (2) model studies of the Rotaflail 
principle and resulting design conclusions, 13 and 
(8) consultation and cooperation with Army 
Ordnance on the design and testing of larger 
Rotaflail models. 0 

In the demolition phase of the problem, Sec¬ 
tion 17.1 activity was confined mainly to the 
development of techniques and methods for 
evaluating demolition clearance devices. This 
included development, calibration, and appli¬ 
cation of indicator mines and a general study 
of the blast properties of anti-tank mines. Con¬ 
sulting aid and design work also contributed 
to a few of the Engineer Board mine-clearance 
design projects. 

a Mathematics Panel. 

b By Section 17.1. 

c By Section 17.1. 


Technical Accomplishments 

In this section a brief summary is given of 
the useful results obtained from the Section 17.1 
program. 

The Rotaflail and its Modifications 

The Rotaflail consists of a large rotating 
drum, mounted in front of a tank, carrying 
flails which beat the ground. The bottom of 
the drum is close to the ground so that at the 
instant of impact with the ground the flails are 
substantially horizontal and moving vertically. 
Model studies were completed on the motion 
of the rotational system leading to conclusions 
on optimum design characteristics. The mag¬ 
nitude and distribution of the impact forces on 
the ground with particular reference to their 
efficiency in detonating land mines were also 
investigated. This work resulted in the follow¬ 
ing conclusions: (1) since there is a wide 
spread in the energy absorbed by individual 
mines under impact of a flail, generous factors 
of safety must be introduced to make sure that 
mines are not left undetonated; (2) when a 
Rotaflail with a 3-ft drum is used against 
TMi-43 indicator mines buried 4 in. deep, a 
drum rotational speed of approximately 125 
rpm is desirable; this leads to a calculated 
power dissipation of 220 hp by 21 flails; (3) 
effectiveness in mine clearing decreases with 
increasing clearance between the bottom of the 
drum and the ground, although this is not 
critical; (4) a Rotaflail operating at the above 
drum speed is considerably more effective than 
a tank tread in detonating mines; (5) an 8-ft 
flail is effective to a distance of 5 or 6 ft from 
the tip of the flail, and this decreases directly 
with drum diameter; (6) studies of a self- 
rotating Rotaflail (“lawn mower”), as con¬ 
trasted to a Rotaflail driven by a power take-off 
from the main tank motors, indicate that it is 
not so satisfactory as the independently driven 
type. 

In addition to studies of the Rotaflail prin¬ 
ciple, investigations were undertaken of a 
slightly different type of device known as the 
Springflail. The Springflail, suggested by the 
Ordnance Department, ASF, eliminates the 
need for a large drum in front of the driving 


CONFIDENTIAL 




48 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


tank by mounting the flails on resilient arms 
(flat springs) which project from a small ro¬ 
tated axle. The axle can be close to the ground; 
the springflails flex as they pass under the axle 
along the ground. The springs and flails then 
straighten out radially as they travel around 
for the next impact because of centrifugal 
forces on the flails and the tendency of the 
springs to straighten. When these conditions 
are satisfied, the flails again are substantially 
horizontal and moving vertically at impact. 
Model studies of the Springflail have led to the 
conclusion that it should be a workable device; 
a full-size model might easily be superior to 
the Rotaflail. It was recognized, however, that 
the Springflail is a more complicated device; 
there are more variables in its design and its 
operation is somewhat more critical. The ad¬ 
vantages offered by the Springflail appear to 
be: (1) less weight, (2) less obstruction to the 
vision of the operator, (8) less target area 
presented to gunfire and mine blasts, and (4) 
higher rotational and forward speeds. 

Investigation was also made of a second mod¬ 
ification of the Rotaflail comprising the use of 
rigid extensions on the drum with the flail 
chains connected to the extremities of the ex¬ 
tensions. It was thought that this technique 
might make possible a reduction in drum diam¬ 
eter. Tests of a model indicated that if such a 
modification should operate satisfactorily, it 
would most likely be critical to speed, flail 
length, and weight under normal conditions. 

Indicator Mine Developments 

In order to evaluate the effectiveness of a 
minefield-clearing device of the shock impulse 
type, a method must be found for accumulating 
sufficient data to obtain the probabilities of 
detonating various types of mines buried under 
a variety of conditions. One method developed 
to secure these data is to measure some quan¬ 
tity characteristic of the reaction of the mine to 
shock impulse, using an indicator mine of such 
type that the probabilities of detonating various 
types of enemy mines can be computed from 
the indicator mine data. Two instruments of 
this type, the TMi-43 indicator mine and the 
universal indicator mine, were developed by 
Section 17.1. In order to extend the range of 


the original universal indicator mine (Ml) to 
include more “blast-proof” mines, an M2 modi¬ 
fication was also designed. Various known 
enemy mines were calibrated against the indi¬ 
cator mines, permitting data accumulated with 
them to be interpreted in comparison with other 
mines under the same experimental conditions. 
The indicator mine developments proved to be 
powerful factors in increasing the reliability 
of results and markedly decreasing the amount 
of work required to evaluate minefield clear¬ 
ance devices. 

Extensive measurements of the dynamic 
characteristics of enemy anti-tank mines were 
carried out in the course of the indicator mine 
program. These measurements established ex¬ 
perimentally the basis for an indicator mine, 
namely, that the condition for detonating a 
shear-pin type mine can be expressed in terms 
of the total energy absorbed by the mine, a 
quantity which is substantially independent of 
the nature of the impulse. Detailed studies of 
anti-tank mine characteristics also made pos¬ 
sible the expeditious design of replica models of 
enemy mines. In some instances these replicas 
were used in the field only until the mines had 
been calibrated in terms of the universal indi¬ 
cator mine; while in other cases, when exam¬ 
ination showed that the principle of operation 
of an enemy mine was substantially different 
from that of the universal mine, the replica 
mines had to be planted in addition to indicator 
mines. (Usually a sufficient number of enemy 
mines were not available for demolition tests to 
be carried out directly with them.) 

The Effect of Point Charges 
On Anti-Tank Mines 

In conducting the program for the calibration 
of the universal indicator mine, a large number 
of tests were made using demolition point 
charges/ 1 The data obtained from these tests 
have been analyzed and an empirical formula 
devised which allows the behavior of the uni¬ 
versal mines to be predicted for a wide variety 
of test conditions. In addition, a method has 
been worked out whereby this formula can be 
applied to a number of foreign mines, usually 

d In much of this work Division 2, NDRC, gave in¬ 
valuable assistance. 


CONFIDENTIAL 




SUMMARY 


49 


of the shear-pin type (e.g., TMi-43, Dutch 
Mushroom Top, Japanese Yardstick, and 
French Light Anti-Tank). Some success was 
realized in correlating indicator mine data with 
mines differently fuzed (e.g., Japanese J-13 and 
J-16 anti-boat mines). Probabilities of detona¬ 
tion can be found for the indicator mine and 
mines related to it under the action of bare 
(uncased) point charges and other types of 
point charge whose action can be expressed in 
terms of an equivalent charge weight of cast 
TNT. A further result is that data obtained 
with a universal indicator mine can be used to 
determine how variations in ground conditions 
and method of burial affect the probability of 
detonation. 

Shock Tube 

A shock tube e was constructed and calibrated 
which permits the study of dynamic character¬ 
istics of anti-tank mines under highly con¬ 
trolled conditions. By using the shock tube, 
variations in ground conditions which are ex¬ 
tremely difficult to control in the field are prac¬ 
tically eliminated, and a more uniform, repeat- 
able shock wave can be produced. The present 
tube is limited to peak pressures in the shock 
wave of less than 150 psi. 

224 Evaluation 

Rotaflail and Springflail 

Model studies of two types of flail mine ex¬ 
ploders have been completed and full-scale tests 
made on a prototype model of one of these, 
the Rotaflail, in cooperation with the Ordnance 
Department. No data resulting from the evalu¬ 
ation studies of the Rotaflail indicate that a 
workable device cannot be made. The inference 
is, rather, that the basic principle is sound and 
that the design and construction of true proto¬ 
type models is simply a matter of straight¬ 
forward engineering. At the cessation of hostil¬ 
ities two Rotaflail prototype models were under 
construction by the Ordnance Department. The 
investigation of the Springflail, while incom¬ 
plete, seemed to indicate that this design prin- 

e Developed from models originated by and designed 
with the cooperation of Division 2, NDRC. 


ciple is promising and could lead to the con¬ 
struction of a mechanical clearing device 
superior to the Rotaflail. 

The Rotaflail and Springflail principles both 
appear to have advantages over other types of 
flail exploders, such as the British Scorpion; 
and it is recognized that on the basis of combat 
usage the British flail exploders have been far 
superior to disk roller and other similar ex¬ 
ploders developed in this country. In the Rota¬ 
flail and Springflail an extended length of flail 
comes in contact with the ground ahead of the 
drum, whereas in the Scorpion only the tip of 
the flail strikes the ground at a point almost 
directly below the rotating drum. One is a beat¬ 
ing action, the other a digging action. This dif¬ 
ference accounts for these apparent advantages 
of the Rotaflail type: (1) less damage is likely 
to be inflicted to the drum if the explosion of a 
detonated mine occurs in front of rather than 
under the drum; (2) there is less probability 
of dirt and unexploded mines being thrown 
back on the pushing tank with the beater-type 
action; (3) the beating action is more efficient 
in detonating buried mines because the impulse 
is delivered vertically. The principal disadvan¬ 
tage of the Rotaflail is its requirement of a 
larger drum; a 5-ft diameter would be ideal 
for flailing action, but such a large diameter 
restricts the visibility of the tank driver. A 
compromise to a 3-ft diameter has been made 
in the models being constructed by Ordnance 
at the time of writing, the model studies indi¬ 
cating that this should give satisfactory flail 
performance. The Springflail does not have this 
same drum-diameter limitation. 

The experimental investigation carried out 
by the Section 17.1 contractor 11 appears to have 
been completed efficiently and soundly. How¬ 
ever, since the investigation was initiated in 
the fall of 1943, the question may be raised 
appropriately as to why, in view of the prom¬ 
ising results, the development as a whole did 
not come nearer to actual combat use. This may 
be ascribed directly to delays on the part of the 
Ordnance Department, ASF, for which no sat¬ 
isfactory explanation is available. In any case, 
their efforts were concentrated on heavy rollers, 
and the promising results obtained with the 

f Carnegie Institute of Technology, Pittsburgh, Pa. 


CONFIDENTIAL 





50 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


Rotaflail model were ignored until the informa¬ 
tion was brought to the attention of Headquar¬ 
ters, ASF. A directive was issued, and a belated 
effort was made to build models in compliance 
therewith. 

It should be noted that all known mechanical 



FLAILING 

PERIOD 





Figure 1 . Theoretical action—ideal case. 



mine-clearing devices have limitations which 
may restrict their future use operationally. 
Countermeasures for this equipment were de¬ 
veloped by the Germans and placed in use. The 
simple expedient of occasionally planting land 
mines containing large explosive charges (for 
example, 200 lb) would make the advisability 


of employing mechanical mine exploders ex¬ 
tremely doubtful. This was in fact the situation 
in the Japanese theaters as the Japanese planted 
many charges considerably larger than that 
contained in an ordinary anti-tank mine (ap¬ 
proximately 10 lb). Although the flail-type mine 
exploders are believed to be the most promising 
mechanical type under development, it is there¬ 
fore questionable whether they offer much hope 
as a solution to the land-mine problem. They 
were, however, cheaper than tanks and could 
have been extremely useful even though an oc¬ 
casional unit was lost as a result of enemy coun¬ 
termeasures. 

Anti-Tank Mine Studies and the 
Indicator Mine 

The development and application of indicator 
mines to the evaluation of explosive mine-clear¬ 
ing devices is believed to be a major accomplish¬ 
ment. The indicator mine has proved to be much 
more useful than was initially expected. With¬ 
out this device and the associated information 
obtained with it, it would have been virtually 
impossible for the Army to conduct an exten¬ 
sive demolition clearing program. 

The auxiliary technique of the shock tube 
is one which appears to be exceedingly promis¬ 
ing and one which has not been applied exten¬ 
sively. Studies with it to date establish it as 
a powerful and useful tool for investigating 
demolition clearance of anti-tank mines and 
analyzing their characteristics. 

The efficient and timely conduct of these 
phases of the program was in a large measure 
due to the ability and resourcefulness of the 
Section 17.1 contractor. g 


23 FLAIL-TYPE MECHANICAL MINE 
EXPLODERS 

In this chapter experimental investigations 
of the Rotaflail, its various modifications, and 
the Springflail are discussed in greater detail. 
The comparative advantages of a self-rotating 
Rotaflail and a Rotaflail independently driven 
by a power take-off are considered, as are the 

e Gulf Research and Development Company, Pitts¬ 
burgh, Pa. 


CONFIDENTIAL 








FLAIL-TYPE MECHANICAL MINE EXPLODERS 


51 


advantages and limitations of wholly flexible 
flails and flails connected to rigid extensions 
from the central rotating drum. 




Figure 2. Arrangement of apparatus. 


231 The Rotaflail 

The Rotaflail consists of a large drum carry¬ 
ing chains or cables which act as flails, the 
behavior of which is illustrated in Figure 1. 
The drum is turned and at the same time moved 


Panel. It will be noted that at the instant of 
impact, when the bottom of the drum is close 
to the ground (see Figure 1), the flails are 
nearly horizontal and moving vertically. This 
action is quite different from that of flail-type 
mine exploders previously constructed in this 
country and in Britain. In these a much smaller 
drum is utilized, with a correspondingly larger 
clearance between the bottom point of the drum 
and the ground. In these devices the flails at 
the moment of impact are substantially vertical 
and moving horizontally. Thus only the tips of 
the flails are effective on buried mines by a 
digging action. 

The advantages visualized for the Rotaflail 
principle and therefore responsible for the in¬ 
vestigation were: (1) detonation of the mine 
in front of rather than under the flail drum; 
(2) a decreased tendency to throw dirt and 
mines back on to the pushing tank; and (3) 
a more efficient and effective transmission of 
kinetic energy from flail to buried mines. 

Initial model studies of the Rotaflail were 
made to substantiate experimentally the con¬ 
clusions of the mathematical analysis and to 
investigate further the practicability of the 



Figure 3. Partial full-scale Rotaflail (left) with chain flails and (right) with loaded rope flails. 


forward at such a speed that the bottom of the 
drum has a small forward velocity with respect 
to the ground. This type of flail action was first 
suggested by Brown University and is de¬ 
veloped in a report of the Applied Mathematics 


idea. Early tests were conducted to analyze 
how the trajectories depend upon the type of 
flail, the rotational speed of the drum, the speed 
of translation, and the clearance between drum 
and ground. A schematic diagram of the ap- 


CONFIDENTIAL 






























52 MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


paratus utilized in this work is shown in Figure 
2. The axle of the drum to which the flail is 
attached does not move and the ground is 
simulated by a moving belt. The drum of the 
model was 10.72 in. in diameter. 

The results of the first model tests, 2 which 
were completed in December 1943, were suffi¬ 






-s 


K 

COD 

IE: 






• 

o 

X 

-ENERGY ABSORBED BY MINE 
-AVERAGE ENERGY OF 5 MINES 
FOR ONE DRUM SPEED 
-MEDIAN ENERGY OF 5 MINES 

• 


r 






( 

' c 

U 


--TMi 

-43 









--TMi 

-42 





/ 

A 

i 





' > 


* 

r 




—NO 

_1 

INDICATIC 

1_1 

IN—► 

¥\ 







10 20 30 40 50 60 70 80 

DRUM ROTATIONAL SPEED IN RPM 


Figure 4. Energy absorbed by indicator mine 
buried 5 in. vs drum rotational speed. Drum 6 
in. above ground. 


ciently promising to enlist the cooperation of 
the Ordnance Department in conducting field 
tests with a partial full-scale model. This series 
of tests 3 was conducted in August 1944 at Aber¬ 
deen Proving Ground with the model shown in 
Figure 3, and good results were obtained, as 
indicated by the conclusions drawn. 

Flailing Action 

The instantaneous shapes and motions of the 
flails of the partial full-scale model were found 
to be in good agreement with those of the small- 
scale models and these, though showing some 
irregularities, were satisfactory. 

Strength of Impact 

A sufficient impulse was found to be delivered 
by the Army standard chain flail to detonate 
a German Tellermine (TMi-43) buried 10 in. 


Active Length of Flail 

Slightly more than half the extended length 
of the flail on the ground was effective in 
detonating anti-tank mines. 

Effect of Explosion on Partial 
Full-Scale Model Rotaflail 

A standard M1A2 high-explosive anti-tank 
mine detonated by the full-scale Rotaflail had 
but little effect on the drum, flails, tank, or tank 
personnel. 

After these test results were reported, the 
Commanding General, ASF, directed the Chief 
of Ordnance to construct two full-scale models 
of the Rotaflail. The Ordnance Department re¬ 
quested NDRC to make further studies and 
asked for cooperation in other tests with the 
partial full-scale model at Aberdeen. Conse¬ 
quently a second series of tests 4 was held at 
Aberdeen in May 1945. In these tests particular 



DRUM ROTATIONAL SPEED IN RPM 

Figure 5. Energy absorbed by indicator mine 
(average of 5 mines) vs drum rotational speed 
for various drum-ground clearances. 

attention was paid to power requirements for 
operating the flail and other possible design 
limitations. 

Energy Absorbed by Mines 

A wide variation was found in the total 
energy absorbed by buried mines under action 


CONFIDENTIAL 

















































FLAIL-TYPE MECHANICAL MINE EXPLODERS 


53 


of a flail, as indicated in Figure 4. With this 
wide variation in energy, which can be ex¬ 
plained by a number of suppositions, generous 
factors of safety must be introduced, especially 
if averages are used, to make sure that mines 
are not left undetonated. 


Effective Drum Speed 

The present tests indicate that with TMi-43 
mines buried 5 in. deep and using a Rotaflail 



0 3 6 9 12 15 18 21 

HEIGHT OF DRUM ABOVE GROUNO 


Figure 6. Energy absorbed by indicator mine 
(average of 5 mines) vs drum-ground clearance 
for various drum rotational speeds. Drum di¬ 
ameter—5 ft. Mines buried 5 in. 

with 5-ft drum diameter 6 in. above the 
ground, a speed of 70 rpm would be desirable; 
at 7-in. burial depth—75 rpm; and at 10-in.— 
80 to 85 rpm. With a larger clearance between 
the drum and ground, the speed should be 
increased. 

Effective Clearance 

The relationship between the energy ab¬ 
sorbed by mines at a depth of 5 in. and drum 
clearance is shown in Figures 5 and 6. (The 
validity of these curves is open to question be¬ 
cause the data are not sufficiently numerous; 
however, the direction of change is reasonably 
certain.) From these figures it is readily con¬ 
cluded that the drum should be operated as 
close to the ground as possible. 

Effect of Mine Depth 

Figure 7 shows how the effectiveness at dif¬ 
ferent speeds varies with the depth at which 


the mines are buried. At 75 rpm, for example, 
TMi-43’s at 5 and 7 in. would be detonated, 
but perhaps only half of those at 10 in. (It 
should be noted here that these depths of burial 
are considerably in excess of those obtained 
with other types of mechanical mine ex¬ 
ploders.) The tests further indicated that a 
Rotaflail turned at the speeds mentioned is 
considerably more effective than tank treads in 
detonating buried mines. This is particularly 
true if the ratio of drum rotational speed to 
translational motion of the tank is chosen such 
that a buried mine has a high probability of 
being hit more than once. 

Recommendations Concerning 
Design of Prototype Mine Exploder 

On the basis of field and laboratory tests a 
number of conclusions have been drawn rela- 



20 30 40 50 60 70 80 

DRUM ROTATIONAL SPEED IN RPM 


Figure 7. Energy absorbed by indicator mine 
(average of 5 mines) vs drum rotational speed 
for various depths of mine burial. Drum-ground 
clearance—6 in. 

tive to the construction of a first prototype 
model. 

1. A Rotaflail with a 3-ft diameter drum 
of width equal to the overall driving tank should 
be constructed. (The choice of 3 ft for the 
diameter is a compromise with visibility—the 


CONFIDENTIAL 





















































































54 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


larger the diameter the poorer the visibility for 
the tank driver and the better the flail action.) 

2. The flails should be half wire cable and 
half chain, should weigh 7 lb per ft, and 
should be 5 ft long. The flails should be spaced 
in planes no more than 6 in. apart. 

3. The drum should be rotated at 125 rpm, 
for which the calculated power into the 21 
flails is 220 hp. This speed should be adequate 
for clearing TMi-43 mines 7 in. below the 
surface of the ground. 

4. The clearance of the drum above the 


The study of the self-rotating Rotaflail 
indicated that although it may be mechani¬ 
cally simpler, it has inherent disadvantages, 3 
namely: (1) the forward velocity must not fall 
below a critical value, which for the dimensions 
and flails considered is about 16 mph; (2) at 
least four flails must be spaced around the 
periphery of the drum to get adequate coverage 
along any line of advance; (3) the power re¬ 
quired to turn the drum would then be four 
times that required for the separately driven 
Rotaflail (however, it should be noted that the 
ground would be covered four times as fast) ; 
(4) difficulty might be expected in supplying 
the power through the ordinary traction be¬ 
tween drum and ground; (5) irregularities in 
the level of the ground would tend to make the 
drum bounce, further aggravating the traction 
difficulties. 

The Use of Rigid Extensions 

A possible method of obtaining a better com¬ 
promise between the use of a small drum for 



Figure 8. Rotaflail model with rigid extension. 

ground should be as small as practicable— 
about 6 in. 

The Self-Rotating Rotaflail 

The above discussion applies to Rotaflails 
independently driven by either a power take-off 
from the main tank engine or from an auxiliary 
engine. Consideration has been given to a 
mechanically simpler type of flail in which the 
drum is rotated merely by being in contact 
with and pushed along the ground. This simpli¬ 
fication would make unnecessary an extra 
engine, transmission, and long drive shaft to 
power the drum. 


Figure 9. Laboratory model of Springflail. 

good vision and long flails for keeping the ex¬ 
plosions remote from the drum is to mount 
rigid extensions on the drum at the end of 
which the flails are attached. A small-scale 
model of such a flail is shown in Figure 8. In 
the laboratory model shown, the relative size 
of the extension was such that a prototype 
might utilize a 3-ft drum and have flails 
mounted on 12-in. extensions. The laboratory 



CONFIDENTIAL 




FLAIL-TYPE MECHANICAL MINE EXPLODERS 


55 


model was found to operate unsatisfactorily 
except at certain critical speeds, flail lengths, 
and weights. It was concluded, therefore, that 
the effectiveness of flails designed with rigid 
extensions might be seriously reduced in opera¬ 
tion if the flail ends were blown off or if mud 
stuck to the flails and changed their linear 
density. Furthermore, little usable length of 
flail is gained by the procedure. 


2 32 The Springflail 4 

The Springflail laboratory model shown in 
Figure 9 consists of a 10-in. chain flail attached 
to a 2.5-in. rotating hub with a 12-in. flat spring 
able to flex in the plane of rotation. The spring 
is made up of several leaves which are adjust¬ 
able in length. The ends of the leaves are 
loosely strapped together and to the center 
leaf so that the spring is an integral unit, but 
the leaves can slide independently as it bends. 
In use the hub is mounted close to the plane of 
impact so that the spring must bend after each 
impact. The hub on the model can be offset 
and oriented with respect to the regular axis 
of rotation, allowing the testing of a partly 
rigid and partly resilient flail support arm for 
the simulation of springs mounted at an angle 
(but within the plane of rotation) on a rotating 
crank. This latter mode of operation appeared 
to offer possibilities in the reduction of spring 
stresses without sacrificing effectiveness. 

The Springflail, although the studies are in¬ 
complete, appears to be a workable type of mine 
detonator. Tests of the model suggest the pos¬ 
sibility that its effectiveness may be made equal 
to that of the Rotaflail, and it would offer some 
advantages, such as less weight, less obstruc¬ 
tion to the vision of the operator, less target 
area to counterfire, and higher rotational and 
forward speeds. The higher forward speed is 
due to the lower relative angular velocity be¬ 
tween the flail and the hub. On the Rotaflail 
the angular velocity of the flail, while it is 
unwinding, is twice that of the drum, 1 but it 
is somewhat less than twice the hub speed on 
the Springflail the instant before impact. Thus, 
to obtain similar flail kinetic energies the hub 
speed of the Springflail must be greater than 


the drum speed of the corresponding Rotaflail. 
A greater impact repetition rate and a higher 
possible forward velocity result. 

The model studies also brought out some 
inherent disadvantages in the Springflail. The 
springs scraped the ground as they flexed back 
under the hub; they would probably throw dirt 
back on a driving tank. It is believed that the 
Rotaflail is easier to construct and that it would 
probably be more durable. There is no drum 
surface present on the Springflail to control the 
bouncing of the flail after impact; without this 
control on the model the chain bounces back 
on itself into a small pile. 


23,3 Conclusion 

A flail-type mine exploder, as exemplified by 
the Rotaflail and possibly the Springflail, is 
believed to be the closest approach fo a satis¬ 
factory solution in terms of mechanical clear¬ 
ance. This view is concurred in by officers in 
the Corps of Engineers and Army Ground 
Forces and may be found expressed in the 
minutes of a combined Service meeting called 
by the Office, Chief of Engineers in May 1945 
to review all mine-detection and mine-clearing 
developments. The advantages of the flail-type 
over disk-roller and other mechanical mine ex¬ 
ploders are briefly: (1) its lesser weight 
permits greatly increased mobility, (2) the 
Rotaflail can be expected to detonate mines 
planted at larger burial depths, and (3) it can 
probably be produced more easily and cheaply. 

It is believed, however, that like all mechani¬ 
cal mine exploders the flail-type is limited in 
application. The device is readily counter- 
measured; the expedient of planting an over¬ 
size charge sufficient to demolish the exploder 
would probably negate its use if such charges 
were encountered very often. The Germans 
developed a countermeasure to mechanical ex¬ 
ploders which is quite simple and effective 
and does not require as much explosive charge: 
they planted a fuze-actuating mechanism some 
8 to 10 ft in front of the main explosive charge; 
when the flail or exploder disk actuates the 
fuze, the main charge blows up under the driv¬ 
ing tank. An additional countermeasure in case 


CONFIDENTIAL 



56 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


of flail-type mine exploders would be the design 
and planting of mines having large mechanical 
damping constants so that the quick impact of 
the flail is not of sufficient duration to explode 
the mine. 

It is not to be concluded, however, that the 
above limitations should preclude the comple¬ 
tion of the flail mine exploder developments. 
The effectiveness of countermeasures depends 
to a very large extent on how ably they are 
employed by the enemy. There is no guarantee 
—in fact there is reason to believe otherwise— 
that the enemy will not be proficient in employ¬ 
ing countermeasures at the outset of another 
war. A stronger argument for completing the 
developments is simply that the flail-type ex¬ 
ploders appear to be the most promising of any 
known type of mechanical mine-exploding 
equipment, and therefore should be completed 
as representative of the best in a category. 
It is also possible that future development 
work will result in uncovering means of re¬ 
ducing the effectiveness of countermeasures. 


24 DEMOLITION CLEARANCE 

TECHNIQUES 

2,4,1 Introduction 

The Engineer Board, Fort Belvoir, Virginia, 
and the JANET Board, Fort Pierce, Florida, 
were the agencies in this country principally 
responsible for the development of equipment 
for clearing lanes through minefields by demo¬ 
lition methods. A description of the specific 
development projects may be found in pub¬ 
lished reports of these boards. 14 Section 17.1 
was able to contribute to a minor extent to 
the design of certain of this equipment, but its 
major contribution was the development of 
techniques for evaluating demolition clearing 
devices and the analysis of the effects of demo¬ 
lition charges on anti-tank mines. 

In evaluating demolition devices the problem 
fundamentally is to determine the effect of 
shock impulses on anti-tank mines, or more 
specifically, to determine the probability of det¬ 
onating various enemy mines as a function of 
distance from explosive charges. At the outset 


it is apparent that this problem involved a 
large number of variables, some very difficult 
to control; to enumerate a few—type of mine, 
depth of burial, soil conditions, atmospheric 
conditions, distance from explosion, explosive 
charge weight, casing, charge form (point or 
linear), and type of explosive. The development 
of methods and techniques for handling these 


3 

CE 

O 

U. 



0 20 40 60 80 100 

DISPLACEMENT IN ' INCH 


Figure 10. Force-displacement characteristic for 
Type TMi-43 anti-tank land mine (steel shear 
pin). 


variables is described, and a brief description 
is given of Section 17.1 contributions to the 
design of demolition clearing equipment. 

Method of Attack 

The general method of attack followed in 
this development may be summarized briefly: 
(1) a study of the static and dynamic charac¬ 
teristics of various types of anti-tank mines 
when subjected to shock impulses in order to 
determine which mine characteristic is least 
dependent on the type of impulse; (2) the 
design and calibration of indicator mines to 


CONFIDENTIAL 















DEMOLITION CLEARANCE TECHNIQUES 


57 


measure this characteristic; (3) a determina¬ 
tion of the effect of shock impulse from point 
charges on an indicator mine, and the correla¬ 
tion of this effect with the behavior of other 
types of mines. 


Measurements of the Dynamic 
Characteristics of Anti-Tank Mines 


A great majority of the anti-tank mine 
studies are similar in their principles of opera¬ 
tion. When a force is applied to the spider of 
the pressure plate, the gap between the under 
side of the plate and the firing piston of the 
fuze closes, and the force is transmitted to the 
firing piston. When the force is sufficient to 
shear the shear pin in the percussion-type fuze, 
or to break the glass ampoule in a chemical 
fuze, the mine detonates. A theoretical study 
of the mine’s dynamic characteristics is not 
easily made since the elastic limit of the ma¬ 
terials of which the mine is constructed is 
generally exceeded, and an unknown amount 
of friction is present; initially, therefore, the 
experimental approach was followed. 

Measurements were carried out on the fol¬ 
lowing types of fuzes or mines: 


Type Mine or Fuze 


Type Measurement 


American M1B1 fuze 
American M1A2 fuze 
American M4 fuze 
German TMi-42 mine 
German TMi-43 mine 
American T6-E1 mine 
Japanese J-93 mine 
Japanese J-13 fuze 
Japanese Type 3 fuze 
Japanese Yardstick fuze 
Dutch Mushroom fuze 
French Light fuze 
Japanese Type A-2(a) fuze 
Japanese Type A-3(a) fuze 
Experimental T6-UN mine 
Experimental S TMi-43 mine 
Experimental S UN mine 
Canadian indicator mine 
Universal indicator mine Ml 
Universal indicator mine M2 


Static and dynamic 

Static and dynamic 

Static and dynamic 

Static and dynamic 

Static and dynamic 

Static and dynamic 

Static and dynamic 

Static 

Static 

Static 

Static 

Static 

Static 

Static 

Static and dynamic 
Static and dynamic 
Static and dynamic 
Static and dynamic 
Static and dynamic 
Static 


Static Tests 

Static tests were made with a hydraulic 
testing machine. The fuze or mine to be studied 
was set in the machine and the force required 


for a given displacement was recorded. A 
sample force-displacement characteristic for a 
German TMi-43 (Tellermine-43) is shown in 
Figure 10. A complete description of the test 
procedure and results is to be found else¬ 
where. 5 - 6 


Dynamic Measurements 

The dynamic characteristic of a mine was 
studied by subjecting the pressure plate of the 
mine to various shock impulses. The force 
during the impulse was measured by means of 
a quartz crystal stack, an amplifier, and a 
cathode-ray recorder. The displacement of the 
pressure plate was measured by means of the 
motion of a coil placed in a magnetic field, an 
amplifier, and a suitable recorder. A photo¬ 
graph of the force-time function was obtained 
with one recorder and simultaneously a photo¬ 
graph of the force-distance function with a 
second. The value of the impulse was then found 
directly by measuring the area under the force¬ 
time function, and the value of the energy 
absorbed by the mine was found by measuring 
the area under the force-distance function. Dif¬ 
ferent impulse forms 5 were obtained by drop- 
weight tests in which various types and sizes 
of weights were used, and by blast tests carried 
out on a laboratory scale with standard 
engineer blasting caps. 

From these data the following conclusions 
could be drawn: (1) for a given type of mine 
the minimum energy required to detonate the 
mine is substantially independent of the type 
of shock impulse acting on the pressure plate; 
(2) for a given mine there is a definite rela¬ 
tionship between absorbed energy and the 
maximum displacement of the pressure plate— 
the minimum energy required to detonate the 
mine is a point on the energy-displacement 
curve such that if the energy is less than this 
amount the mine does not detonate, and if 
greater the mine detonates; (3) for the drop- 
weight tests in which the test weight made a 
“perfect hit” h on the impact plate, there is a 
linear relationship between the energy ab¬ 
sorbed and the kinetic energy of the weight. 

h A perfect hit occurs when the test weight flat 
strikes the impact plate and the weight bounces approx¬ 
imately straight up, indicating that the flat was parallel 
to the plate at the moment of impact. 


CONFIDENTIAL 




58 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


2.4.3 Indicator Mine Developments 

At the time the drop-weight studies were 
being made, the Engineer Board urgently 
needed a device which could be used to evaluate 
the effectiveness against German mines of 
demolition methods of minefield clearance. The 
drop-weight studies showed that, among the 
mines available for study up to this time, the 
German TMi-43 was one of the most difficult 
mines to detonate with shock impulses. In view 
of this the decision was made to use the TMi-43 


PRESSURE PLATE 



Figure 11. The TMi-43 indicator. 

as a temporary indicator mine. On the basis 
of engineering data supplied to the Engineer 
Board, this German mine was substantially 
duplicated in the form known as the TMi-43 
indicator. A method was devised so that this 
mine could be used as an indicator for shock 
impulses which were less than or equal to the 
minimum necessary to detonate the mine. In 
practice it was found that the range of this 
indicator was too small to give information 
on types of mines which differed appreciably 
from the TMi-43. Therefore the universal indi¬ 
cator mines Ml and M2 were designed. 

The universal indicator mine Ml had an 
energy range approximately twice that neces¬ 
sary to shear the pin in the TMi-43 indicator. 
When studies of anti-tank mines were extended 
from German to Japanese types, it immediately 
became apparent that the Japanese mines were 
considerably harder to detonate by shock im¬ 
pulse. The characteristics of some Japanese 
mines were such that the range of the uni¬ 
versal indicator Ml was insufficient to handle 
them. Accordingly the M2 universal indicator 
mine was developed; the M2 is simply a minor 


modification of the Ml in which the pressure 
plate on the Ml is replaced by a smaller and 
stronger one. 

Apparatus Description 

The TMi-43 indicator (see Figure 11) con¬ 
sists of three parts: the pressure plate, the base, 
and the fuze. The fuze is inserted in a fuze well 
in the base, and the pressure plate is screwed 
on tightly. The pressure plate (spot-welded 
type) acts as a spring with a constant for 
uniformly applied forces of about 3,000 psi and 
an elastic limit of about 250 lb. There is a gap 
between the under side of the pressure plate 
and the top of the firing pin (rounded knob in 



Figure 12. Measuring the depression of the knob 
by the dial method. 


Figure 11) of the fuze. The firing pin is held 
in place by the shear pin visible at the top of 
the fuze. When a force is applied to the pres¬ 
sure plate, the gap closes so that the force 
acts on the shear pin. A measurement of the 


CONFIDENTIAL 







DEMOLITION CLEARANCE TECHNIQUES 


59 


depression of the rounded knob can be related 
to the energy absorbed by the mine from the 
shock impulse. Thus the depression of the pin 
can be related to the probability of detonating 
other shear-pin mines which are operated by 
impulses less than that required to shear the 
pin in the TMi-43 indicator. A dial method of 



Figure 13. Phantom view of universal fuze. 


measuring the depression of the knob is shown 
in Figure 12. 

The universal indicator mine Ml comprises 
an ingeniously designed universal indicator 
fuze and a modified TMi-43 indicator mine base 
and pressure plate. The modification consisted 
of enlarging the fuze well and pressure plate 
bushing. A phantom view of the universal fuze 
is shown in Figure 13 (about twice size). It 
consists of a piston which receives the force 
from the pressure plate, specially designed 
Belleville springs, a fuze body which sets in a 
fuze well in the mine base, a measuring pin, 
and a chuck which holds the measuring pin. 
Initially the measuring pin is set flush with 
the top of the piston. When a force is applied 
to the piston the springs deflect, the piston 
moves down, and the measuring pin moves 
down the same amount. When the force is 


removed, the springs return to the initial posi¬ 
tion, but the measuring pin is held by the chuck 
in the deflected position. A dial gauge with 
adapter is used to measure the deflection of 
the measuring pin, as shown in Figure 14. The 
load-deflection characteristic of the Belleville 
springs was chosen to simulate shear wire and 
to give an approximately linear energy-deflec¬ 
tion curve in the complete mine. Hence a 
measurement of pin displacement, since this is 
linearly related to the energy absorbed by the 
mine, measures the dynamic characteristic of 
the mine that is substantially independent of 
the type of shock impulse. By resetting the pin 



Figure 14. Measuring the deflection of the meas¬ 
uring pin with a dial gauge and adapter. 

the universal fuze may be re-used an indefinite 
number of times. 

To increase the range of the universal mine 
by a factor of ten to twenty (in order to cover 
such mines as the Japanese J-13, J-16, Type 3, 
and Yardstick) the pressure plate was changed 


CONFIDENTIAL 








60 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


for the M2 model. In the final design the 
diameter of the M2 pressure plate was reduced 
from 5 7 /g in. (the diameter of the Ml) to 2% 
in., and the thickness of the material was in¬ 
creased from 28 to 24 U. S. gauge. This pro¬ 
cedure is obviously satisfactory, since the fuze 
is a force-measuring device and the mine as 
a whole is a pressure-measuring device. 

Calibration 

The first calibration studies were carried out 
by means of drop-weight tests on unburied 
mines. In these calibrations a correction must 
be introduced for pressure plate area. As this 
correction was thought likely to be in error, 
and because it is particularly difficult to esti¬ 
mate effective pressure areas for some mine 
pressure plates, extensive calibration programs 
were carried out using explosive charges at 
Fort Belvoir, Virginia, and the Engineer Board 
Field Stations at Port Royal, Virginia, and 
Vero Beach, Florida. The field tests early 
showed that, without special precautions, the 
scatter in the data is so large that the calibra¬ 
tion value can be determined only with a large 
amount of data. Since only a small number of 
enemy mines were available for the experi¬ 
ments, extensive precautions 6 were taken to 
make test conditions as uniform as possible. 

To calibrate a given mine, fields sown with 
both the mine and universal indicator are sub¬ 
jected to blast from the same charge. The 
calibration value is the average reading D p of 
universal indicator mines planted at a distance 
at which 50 per cent (or some other chosen 
fraction) of the subject mines are exploded. 
For this calibration to be meaningful the read¬ 
ing must be independent of the weight of 
charge, the type of soil, and the depth of burial 
of the mines; only then can the calibration be 
said to be consistent. 

It has been shown theoretically 7 that it is 
quite easy to construct a mine of the shear-pin 
type that could not be calibrated in a consistent 
manner against universal indicator mines. This 
argument is based on the following reasoning. 
The energy absorbed by a buried mine usually 
depends in some unknown way upon both the 
peak pressure and the total impulse in the 
shock wave. For small charges and with the 


mine buried close to the charge, peak pressure 
is the dominating factor; for large charges and 
with the mine buried at some distance from the 
explosion, total impulse is the most important 


factor. Thus two constants of the mine p«> 
I oo are involved which are defined by 

and 

k P „ = rmdx. 
J 0 Ax ° ’ 


(1) 

/. - Uj“ "W-H 

1 

2 

(2) 


The symbols have the following meanings: 

A is the area of the moving member of 
the mine which is exposed to blast; 

M is the mass of the moving member, 
including the superposed earth cover; 
x 3 is the total displacement of the mov¬ 
ing member required to actuate the 
mine (or to produce a given reading 
on universal indicator) ; 

F(x)dx is the total static work required to 
displace the moving member through 
x 3 units. 

If the two values of /«, and p* computed 
for the average reading D p of the universal 
indicator mine at which the other (or subject) 
mine shears are not the same, then the calibra¬ 
tion will vary with charge weight. Fortunately, 
for most of the mines tested, these readings 
apparently were nearly enough the same so that 
a consistent calibration could be made. 6 


2.4.4 The Effect of Point Charges on 
Anti-Tank Mines 

In conducting the program for the calibra¬ 
tion of universal indicator mines a large num¬ 
ber of tests using point charges were made. 
The data obtained from these tests have been 
analyzed and an empirical formula devised 
which allows the behavior of the universal mine 
to be predicted for a wide variety of conditions. 
In addition a method has been worked out 
whereby this formula can be applied to many 
foreign mines. 6 

This so-called point charge formula gives a 
convenient method for determining the proba- 


CONFIDENTIAL 






DEMOLITION CLEARANCE TECHNIQUES 


61 


bilities of detonating different mines for vari¬ 
ous soil conditions. It has been used to compute 
universal scale graphs for 0-in., 2-in., 4-in., 
and 6-in. depths of burial. The graph for 2-in. 
depth of burial is given in Figure 15. From 
these graphs the distance from the charge cor¬ 
responding to an arbitrarily chosen probability 
of detonating any mine (which has been corre¬ 
lated with the universal indicator mine Ml 
or M2) can be determined for weights of charge 


The versatility of the universal indicator 
mine is shown by the fact that it has been used 
to determine probabilities of detonation for 
enemy mines which are basically different in 
fundamental principle of operation, for ex¬ 
ample, the J-13 and J-16 Japanese anti-boat 
mines. Approximate probabilities of detona¬ 
tion 6 have since been determined also for the 
Japanese Type 3 (Flowerpot) mine, which is 
not of the shear-pin type. 



Figure 15. Curves showing distance (in feet) to charge for given weight and 
universal scale reading. 


from 10 to 1,000 lb. While these graphs apply 
to cast TNT charges (cylindrical, with the 
length twice the diameter) hung at a scaled 
height above the ground (height in feet equal 
to cube root of the weight in pounds), they may 
be applied to other types of demolition weapons 
by using an equivalent weight of charge. Other 
theoretical studies of the effects of point and 
line charges on indicator mines have been car¬ 
ried out by Division 2, NDRC, in cooperation 
with Division 17. 7>8 * 10 


245 Shock Tube 

Introduction 

In the preceding sections it was stated that 
dynamic characteristics of anti-tank mines 
have been studied by means of drop-weight 
tests and that these results have been used to 
calibrate, in terms of shock pressure impulses, 
enemy mines with the universal indicator mine. 
It was also noted that the majority of calibra¬ 
tion data was accumulated in the field using 


CONFIDENTIAL 























































































62 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


point charges. The drop-weight method of cali¬ 
bration is difficult or impossible to employ when 
the enemy mine does not have a well-defined 
flat surface for a pressure plate (e.g., the Jap¬ 
anese Yardstick mine). The field calibration 



Figure 16 . Assembly view of shock tube. 


method is limited by variations in ground con¬ 
ditions so difficult to control that considerable 
data must be taken to establish a definite cali¬ 
bration value. It was therefore suggested that 
a large-scale model of a shock tube developed 
by Division 2, NDRC, for use in calibrating 
pressure gauges would be a useful tool in the 
calibration program as well as for other studies. 
In consultation with Division 2 a shock tube 
of 15.25-in. ID was designed on the basis of 
a theory 11 developed by Division 2. The neces¬ 
sary engineering work was completed, and a 
tube of the above dimensions was built and 
calibrated. (This tube was subsequently given 
to the Engineer Board for further experimental 
testing of mines.) 

Description 

The tube (see Figure 16) comprises a pres¬ 
sure chamber, an expansion chamber, and a 
mine chamber. Compressed gas from an air 
compressor or dry nitrogen tank is put into 
the pressure chamber through a connection 
(see Figure 17). The pressure chamber is 


formed by two aluminum diaphragms clamped 
between the flanges at the top of the tube. A 
round-pointed knife blade is placed in the 
region between the diaphragms and attached 
to an external arm in such a manner that the 
center of the bottom diaphragm may be cut by 
pulling this lever arm down. It has been found 
that, when the chamber pressure is greater 
than or equal to 70 per cent of the static pres¬ 
sure required to burst the diaphragm, the 
diaphragm will burst when cut by the blade. 

The shock wave formed when the bottom 
diaphragm bursts travels down the expansion 
chamber and is reflected by the ground surface 
in the mine chamber. The reflected wave travels 
up the tube to the top diaphragm and is, of 
course, reflected again. This second reflection is 
only about 30 per cent of the amplitude of the 
first and consequently does not cause appreci¬ 
able error in studying the effect of shock 
impulses on mines. A baffle has been built in 
the expansion chamber below the bottom 
diaphragm in order to catch, or at least slow up, 
fragments from the bursting diaphragm. The 
mine chamber at the bottom of the tube rests 
on a hydraulic jack so that it may be removed 
easily. 

Calibration 

A tourmaline crystal pressure gauge was 
obtained from Division 2, and suitable record¬ 
ing equipment was built to make pressure-time 
measurements of the shock wave. Figure 18 
shows a calibration curve of the peak pressure 
in the shock impulse as a function of chamber 
pressure (obtained using the baffle). The cali¬ 
bration studies show that the peak pressures 
repeat in successive tests within a 5 per cent 
experimental error. The time duration of the 
shock impulse obtained for various chamber 
pressures was nearly constant and independent 
of the pressure. Other experiments indicate 
that the pressure at the center of the tube is 
about 5 per cent greater than the pressure 
at the side. 

Mine Tests 

During tests for the calibration program 
mentioned above, mines were placed in the 
chamber in order to determine the effect of 


CONFIDENTIAL 





DEMOLITION CLEARANCE TECHNIQUES 


63 


shock impulses. These tests brought out deci¬ 
sively that the compactness of the soil above 
and below the mine may be responsible for as 
much as a 30 or 40 per cent variation in cali- 


successive tests. Thus comparative tests can 
be carried out with an indicator mine to meas¬ 
ure the following: (1) effect of depth of burial 
in a soil of given type (controlled moisture 



PRESSURE CHAMBER IN PLACE UPPER DIAPHRAGM IN PLACE 


Figure 17. Views of shock tube. 


bration value, confirming results previously 
obtained from the field trials. 

Future Studies Using the Shock Tube 

Sufficient tests have been made using the 
tube to show that the shock impulses repeat in 


content, etc.), (2) effect of different types of 
soil on a mine at a given depth, (3) effect of 
moisture content of soil, (4) effect of density 
of soil, (5) effect of temperature of soil, (6) 
effect of hardness of soil, and (7) correlation 
between indicator and enemy mines. 


CONFIDENTIAL 














64 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


In addition to these studies, shock tubes can 
be used to analyze the effect of impulse on any 
device which is inserted into the chamber. 
Experiments of the type mentioned above have 
not been carried out, as the tube calibration 
was completed just prior to the cessation of 
hostilities. 

The present shock tube is limited to peak 
pressures in the shock wave of less than about 
150 psi. Tests have shown that this maximum 
is not sufficient to detonate the Japanese anti¬ 
boat mine and the Type 3 (Flowerpot) mine. 
It may therefore be desirable to build a new 


been developed by the Engineer Board Field 
Station, Vero Beach, Florida. The device 14 con¬ 
sists of a flexible charge about 300 ft long 
and 3 in. in diameter (4 lb per ft explosive 
weight), a rocket and rocket launcher for 
projecting the charge across a minefield, an 
amphibious sled for storing and transporting 
the charge, and a firing device for first igniting 
the rocket and then detonating the charge. The 
charge is made of composition C-3 and is 
wrapped around a nylon rope that extends 
through the center of the charge. A nylon sock 
is then pulled over the charge and tied to the 



0 100 200 300 40C 

CHAMBER PRESSURE IN PSI 

Figure 18. Peak pressure as a function of chamber pressure in shock tube. Baffle 
in place. 


tube which will go to higher peak pressures. 
It is believed this may be done using the same 
basic design by increasing the wall thickness 
of the tube, flange dimensions, and bolt 
diameters (i.e., by constructing a larger scale 
model). 

Auxiliary Design Studies 

In addition to the investigations described 
heretofore, certain auxiliary design studies 
were carried out in connection with the de¬ 
velopment of demolition clearance devices by 
the Engineer Board and the reproduction of 
certain enemy mines. These studies are sum¬ 
marized briefly in this section. 

Firing Device for Projected Line Charge 12 

The projected line charge [PLC] is a demo¬ 
lition device for clearing minefields which has 


rope at intervals of about 6 in., giving a link 
sausage appearance to the assembly. Initially 
the charge is flaked into the sled under a ply¬ 
wood cover. When the rocket is ignited, a steel 
cable, which connects the rocket to one end of 
the charge, rips off the cover, allowing the 
charge to be dispensed as the rocket travels out. 
After the rocket has burned out and the charge 
has come to rest the charge is detonated. The 
firing device must be able to ignite the rocket 
and then detonate the charge by remote control 
(e.g., from inside a tank). 

A firing device which has the following char¬ 
acteristics was designed for the Engineer 
Board: (1) electrical firing of the rocket and 
charge, using a standard engineer blasting 
machine; (2) complete waterproofing of all 
switches and cables; (3) pull-wire arming 
switches arranged in such a manner that the 
pull wire which arms the charge cannot be 


CONFIDENTIAL 









DEMOLITION CLEARANCE TECHNIQUES 


65 


removed until the wire which arms the rocket 
has been pulled; (4) a safety switch in the 
sled which arms the charge when the rocket 
leaves; (5) with the igniter and detonator 
wired up, the switch circuit shorts both circuits 
prior to actuating the arming levers. 

The Amphibious Snake, M4 13 

The amphibious snake M4, a cooperative de¬ 
velopment of the Army Engineer Board, the 



Figure 19. The amphibious snake (top) on the 
LVT, ( middle) striking the beach, and ( bottom ) 
after coming to rest. 


U. S. Marine Corps, Ordnance Department, 
ASF, and NDRC, is a demolition device for 
use in amphibious landing operations. It con¬ 


sists of a 45-ft section of standard demolition 
snake M3, a rocket motor to drive the snake, 
a nose which causes the snake to plane on the 
surface of the water, and suitable arming, fir¬ 
ing, and safety devices. In Figure 19 the 
amphibious snake is shown mounted on an 
LVT, in operation, and after coming to rest. 

The portions of the development in which 
Section 17.1 was active were: (1) designing 
workable production models of the triple-rocket 
motor-mount assembly originated by the Rocket 
Division, Ordnance Research and Development 
Center, Aberdeen Proving Ground; (2) design¬ 
ing a suitable locking device to hold the snake 
prior to launching from the LVT; (3) design¬ 
ing a nose to control the snake so that a 
straighter course is followed; (4) designing 
motor brackets and an arming device for the 
single-rocket motor developed by Division 8, 
NDRC. 

The snake nose shown in Figure 20 was 
designed by Division 12, NDRC, at the request 
of Section 17.1. The outstanding characteristic 
of this nose is its lateral stability, i.e., it tends 



Figure 20. Views of boat nose. 


to keep the snake traveling on a straight course. 
Field testing of this nose was done with the 
cooperation of Section 17.1. 

Duplication of Enemy Mines 

From time to time during investigations of 
characteristics of anti-tank mines, Section 17.1 
was requested by the Engineer Board and/or 
Army Ordnance to draw up specifications for 


CONFIDENTIAL 







66 


MECHANICAL AND DEMOLITION CLEARANCE OF LAND MINES 


small-scale production in this country of certain 
enemy mines. Among these were the Japanese 
J-13 and J-16 anti-boat mines, the Japanese 
Yardstick mine, the German TMi-43, and the 
Japanese Type 3 (Flowerpot). 

In connection with the duplication of the 
anti-boat mines, a rather extensive program 6 
was undertaken in cooperation with Division 2, 
NDRC, in comparing the reaction to under¬ 
water shock impulses of original Japanese 
mines and the U. S. replicas. 

In many instances it would have been impos¬ 
sible for the Army to study the effect of its 
demolition clearing devices on enemy mines 
were it not for the availability of the replica 
mines, as very few enemy mines were brought 
back to this country. Thus the best substitute 
procedure for conducting experiments with 
enemy mines directly was to use copies which 
simulated as nearly as possible design charac¬ 
teristics of original samples. 


Evaluation and Conclusion 

The indicator mine development and the 
associated programs have proved in practice 
to be much more useful than was initially ex¬ 
pected. This is due primarily to the very 
capable handling and direction of the work by 
the Gulf Research and Development Company, 
Pittsburgh, Pa., under contract with Division 
17. One final result of the investigation, a 
formula which predicts for a wide range of 


explosive charges probabilities of detonating 
various enemy mines buried at different depths 
and in different soil conditions, is believed to 
be a major accomplishment. Without this and 
other contributions described in this chapter, it 
would have been difficult, if not impossible, to 
conduct the very extensive explosive mine- 
clearing program carried on by the Armed 
Services. 

It is difficult to conclude a report such as 
this without considering the prospects of a 
future research program. It is readily apparent 
that the problems involved in demolition clear¬ 
ance of anti-tank mines are so complex that 
they cannot be handled adequately by other 
than trained technical personnel. Since the 
land-mine problem is a continuing one to which 
a solution has not yet been devised, the question 
may be appropriately raised as to what agency 
or agencies will continue the investigation. It 
is assumed that the various branches of the 
Army will continue their programs. It may be 
concluded, however, that this work will not 
progress as rapidly and satisfactorily by itself 
as it would if other technical organizations 
were able and willing to cooperate. For ex¬ 
ample, it is doubtful whether the Army will 
have the personnel and facilities to continue 
investigations made possible by the design and 
construction of the shock tube, and this in¬ 
strument has as yet been exploited only to a 
limited extent. It is strongly recommended 
therefore, that the peacetime agency which ulti¬ 
mately succeeds NDRC include anti-tank mine 
clearance in the program it prosecutes. 


CONFIDENTIAL 



Chapter 3 


MAGNETIC CHARACTERISTICS OF VEHICLES AND 
MAGNETIC LAND MINES 


31 INTRODUCTION AND SUMMARY 

T his chapter describes investigations of the 
magnetic fields beneath and around tanks 
and other motorized vehicles and the develop¬ 
ment of firing devices for land mines which 
are actuated by the magnetic field of a vehicle 
passing over them. The purpose of the investi¬ 
gation was to ascertain how difficult the prob¬ 
lem of degaussing tanks might be and how 
effectively magnetically operated mines might 
be employed against armored vehicles, includ¬ 
ing, as a corollary, investigation of the 
detection of mines containing quantities of 
ferromagnetic material. 

In order to obtain the necessary measure¬ 
ments in connection with the study of the 
magnetic fields of vehicles a sensitive automatic 
recording magnetometer was developed. Six 
units were constructed so that complete meas¬ 
urements of the magnetic fields of vehicles 
could be obtained by driving the vehicles over 
the sensitive units. A study of the measure¬ 
ments showed that strong magnetic fields ca¬ 
pable of discharging mines of suitable design 
are produced at the surface of the ground be¬ 
neath all types of vehicles. The vertical compo¬ 
nent was found most appropriate for operating 
a mine because the maximum values of that 
component occur predominantly beneath the 
vehicle and with a more regular distribution 
than for either the longitudinal or transverse 
components. Degaussing of vehicles is not con¬ 
sidered to be a practical countermeasure be¬ 
cause the fields are so complicated as to require 
complex arrangements of degaussing coils for 
any fairly satisfactory neutralization of fields. 
Also, changes in the currents flowing in the coils 
would be required for each change of heading 
of the vehicle for the effect of degaussing to 
be relatively complete. 

Two different magnetic firing devices for 
land mines were developed. The Department of 
Terrestial Magnetism of the Carnegie Insti¬ 
tution of Washington developed a mechanical 


device, and the Gulf Research and Development 
Company an electronic device. Both are de¬ 
signed so as not to respond to slow changes in 
the earth’s field, but to fire the mine when 
the vertical component of the magnetic field 
changes rapidly by more than a critical value. 
This value can be set as low as 50 milligauss. 
Several units of each device were made and 
after successful demonstrations were delivered 
to the Engineer Board for further study and 
evaluation. 

A detector was developed which can detect 
a ferromagnetic object the size of a land mine 
at a distance of about 4 ft. It has been described 
in Chapter 1 and will not be discussed here. 

3 2 MAGNETIC FIELDS OF VEHICLES 

In order to study the feasibility of using 
magnetically operated mines against vehicles 
and to work out possible countermeasures, de¬ 
tailed information on the magnetic fields of the 
vehicles is needed. The amount of data required 
and the limited availability of vehicles for 
study indicated the use of automatic recording 
instruments. After completion of the instru¬ 
ments, measurements were made at the Aber¬ 
deen Proving Grounds on the fields of eleven 
vehicles. 3 

3,21 Method of Measurement 

The CIW Marine Magnetometer 1 was 
adopted as an automatic recording instrument. 
The sensitive unit of the magnetometer con¬ 
sists of a Permalloy wire 12 in. long and 0.027 
in. in diameter. Around this is wound a single¬ 
layer coil of about 200 turns—100 turns wound 
in one direction and 100 turns in the other 
direction. A separate secondary coil of between 
10,000 and 20,000 turns surrounds the primary 
coil. When the system is properly adjusted, the 
deflection of a galvanometer in the secondary 
circuit on making or breaking the primary 
circuit is proportional, over a range of several 


CONFIDENTIAL 


67 


MAGNETIC CHARACTERISTICS OF VEHICLES AND MAGNETIC LAND MINES 



NOTE: SYMBOL 


DENOTES CONNECTION OF EACH OF SIX UNITS TO COMMON CIRCUIT 


Figure 1 . Electronic circuit of six-element recording magnetometer. 


CONFIDENTIAL 

































































































MAGNETIC FIELDS OF VEHICLES 


69 


hundred milligauss, to the field in the direction 
of the Permalloy wire. a A neutralizing coil com¬ 
pletely surrounding the element can be used 
in several ways. It can be used to neutralize 
part of the field so as to bring the instrument 
into its range of linearity, or it can be used 
to neutralize the entire field, as shown by zero 
deflection of the galvanometer upon making 
or breaking the primary circuit, giving a null 
instrument, with the field being proportional to 


neutralize the earth’s field so that the instru¬ 
ment will read only the field due to the vehicle. 

The instrument can be made continuous- 
reading by exciting the primary circuit with 
half-wave alternating current. This requires a 
commutating device in the secondary circuit in 
order that the output may be read on a d-c 
meter. This is done electronically by applying 
the primary exciting voltage to the grids of a 
pair of triodes connected in push-pull. The 



Figure 2. Complete six-element recording magnetometer. 


the neutralizing current. In the use of the in¬ 
strument described here, the coil is used to 


a A rough explanation of this follows. The Permalloy 
may be considered as being composed of two parts in¬ 
side the two halves of the primary. When the primary 
circuit is open, each half of the Permalloy has a flux, 
in the same direction for each half, which is propor¬ 
tional, over a limited range, to the field H. When the 
primary circuit is closed, the two halves are saturated 
in opposite directions to the saturation flux-density B. 
The changes in the fluxes in the two halves are propor¬ 
tional to (-\-B — kH) and (—B — kH ). The deflection 
of the galvanometer is proportional to the flux change 
in the secondary, which in turn is proportional to the 
sum of the flux changes in the two halves of the 
Permalloy. Since this sum is —2 kH, the deflection of 
the galvanometer is proportional to H. 


ends of the secondary coil are also connected to 
the grids of the triodes. The blocking impulse 
from the primary circuit is sufficient to make 
the tubes cut off during the half-cycle that the 
primary current flows. The difference between 
the plate currents of the tubes, which is propor¬ 
tional to the voltage in the secondary coil and 
thus to the magnetic field, is read on a microam¬ 
meter or recorded on a General Electric photo¬ 
electric recorder. The calibration is usually 1 
ix a for 10 milligauss. The primary excitation is 
obtained either from a vibrator or, preferably, 
from a half-wave Tungar rectifier if 110-v 
alternating current is available. Figure 1 gives 
the circuit of one of the units, and Figure 2 


CONFIDENTIAL 






SCALES IN GAUSS SCALES IN GAUSS _ SCALES IN GAUSS 


70 MAGNETIC CHARACTERISTICS OF VEHICLES AND MAGNETIC LAND MINES 





Figure 3. Magnetic fields beneath an M-4 medium tank for both northerly and southerly headings: 
(top) vertical field, ( middle) longitudinal field, and ( bottom ) transverse field. 


CONFIDENTIAL 


SCALES IN GAUSS SCALES IN GAUSS SCALES IN GAUSS 




































































































































































MAGNETIC FIELDS OF VEHICLES 


71 


is a photograph of the complete six-element 
recording magnetometer. 

The magnetometer elements were mounted 
in six weatherproof brass tubes about 1 in. in 
diameter and 6 in. long. Holes and troughs were 
dug so that the elements could be mounted to 
point east, south, or down at six points spaced 
28 in. apart on an east-west line. The spacing 


was so chosen that, for tanks, elements 2 and 
5 were under the tracks. The drivers were in¬ 
structed to drive at uniform speeds, and marks 
were made on the records at the times of first 
and last contact of the vehicle with the 
elements. Each vehicle was driven on both 
north and south headings for all three com¬ 
ponents of the field. 


A typical set of data is shown in Figure 3, 
in which are plotted the three components of 
the field of an M-4 medium tank for both north 
and south headings. The rectangle represents 
the plan outline of the tank. The six straight 
lines are the lines of measurement and are 
also the base lines of the graphs of the field 
components. 


3 2 2 Analysis of Magnetic Field of M-4 
Medium Tank 

An examination of Figure 3 reveals some 
interesting features. The fields are practically 
zero at a distance of 10 ft before or behind 
the tank, but the records do not extend far 
enough laterally to show this decrease. The 




Figure 4. Contour maps of vertical magnetic field in milligauss beneath an M-4 medium tank when 
(top) heading north and (bottom) heading south. 


CONFIDENTIAL 














































72 


MAGNETIC CHARACTERISTICS OF VEHICLES AND MAGNETIC LAND MINES 


vertical field is much less irregular than the 
horizontal fields and has a considerable value 
over a larger area under the tank than do the 
others. These and other considerations make 
it desirable that magnetic anti-tank mines be 
designed to operate on the vertical magnetic 
field of the vehicle. For this reason, only the 
vertical component of the field will be analyzed 
in any detail. 


z p = i(z n + z s ), 

Zi = i(Z n - Z s ). 

Here Z p and Z { are the permanent and induced 
fields and Z n and Z s are the total fields for north 
and south headings, respectively. The “perma¬ 
nent” field is not permanent in the usual sense 
but is probably largely due to magnetism in¬ 
duced by the earth’s vertical field and would 



Figure 5. Contour maps of vertical fields of an M-4 medium tank: (top) “permanent” field and ( bottom) 
“induced” field. 


A better picture of the vertical field is given 
by the contour maps of Figure 4. The cross- 
hatched regions show where a firing device set 
for 150 milligauss will be actuated. There is 
considerable difference between the fields for 
the two headings of the vehicle. If we assume 
that the total field is the sum of a permanent 
field and an induced field which changes sign 
with the heading of the vehicle, we can write 


change with magnetic latitude. The “induced” 
field is attributed to magnetism induced by the 
earth’s horizontal field, and will be proportional 
to the cosine of the magnetic heading of the 
vehicle. Figure 5 shows contour maps of the 
permanent and induced fields. 

Examination of Figure 5 shows that both 
Z p and Z { are roughly laterally symmetric, while 
Z p is roughly longitudinally symmetric and Z { 


CONFIDENTIAL 































MAGNETIC FIELDS OF VEHICLES 


73 


longitudinally antisymmetric. If we consider 
the fields as being made up of symmetric and 
antisymmetric parts, and neglect the smaller 
part, we will obtain fields which will differ 
from the actual fields by an amount which is 
never larger than 60 milligauss and which will 
be much more susceptible to a theoretical treat¬ 
ment. The results of this process are shown in 
Figure 6, which gives contour maps of the 
laterally symmetric, longitudinally antisym- 


Fourier series expansion and then expressing 
the field due to the model in the same sort of 
expansion. The adjustable parameters involved 
in the model can then be chosen to give the 
best fit between the observed coefficients and 
those of the model. The mathematical details 
of this procedure may be found elsewhere. 33 
Figure 7 shows contours of the current func¬ 
tion obtained in this way which, at a height of 
4 ft, will give at the plane of observation the 



Figure 6. Contour maps showing (top) longitudinally antisymmetric, laterally symmetric “induced” 
vertical field of an M-4 medium tank and ( bottom ) longitudinally and laterally symmetric “permanent” 
vertical field of an M-4 medium tank. 


metric part of the induced field, and the later¬ 
ally and longitudinally symmetric parts of the 
permanent field. 

If degaussing of the tank is to be done, it 
would be convenient to devise a distribution of 
currents or magnetic material above the plane 
of measurement whose magnetic field at the 
plane of measurement shall approximate the 
observed field of the tank. This can be done 
by expressing the observed field as a double 


laterally symmetric, longitudinally antisym¬ 
metric induced field shown in Figure 6. 


3 2,3 Measurements on Other Vehicles 

Measurements of the magnetic fields of ten 
other vehicles were made, but the data were 
not analyzed. If desired, the data could be 
treated in the manner outlined in the previous 


CONFIDENTIAL 



















































74 


MAGNETIC CHARACTERISTICS OF VEHICLES AND MAGNETIC LAND MINES 


section. The profiles of the fields may be found 
elsewhere. 35 The results are summarized in 
Table 1. This table includes the maximum val¬ 
ues of each of the three components of the field 
under the vehicle, the maximum sensitivity re¬ 
quired of a detonator to insure firing no matter 


For all the vehicles the vertical fields are more 
uniform than either the transverse or the longi¬ 
tudinal fields. The distance at which premature 
firing may take place could be greatly reduced 
by decreasing the sensitivity so that fields of 
100 or 150 milligauss would be required for 


Table 1 . Maximum fields produced by vehicles, sensitivity required of mine to assure detonation, and maxi¬ 
mum distance from vehicle at which firing could take place for mine with sensitivity of 50 milligauss. 



Maximum field under 

Sensitivity required of 
mine to assure 

Distance from vehicle at which 
mines of ±50 milligauss sen¬ 



vehicle 



detonation 

sitivity could be fired 

Vehicle 

Ver¬ 

tical 

Longi¬ 

tudinal 

Trans¬ 

verse 

Ver¬ 

tical 

Longi¬ 

tudinal 

Trans¬ 

verse 

Ver¬ 

tical 

Longi¬ 

tudinal 

Trans¬ 

verse 


milli¬ 

milli¬ 

milli¬ 

milli¬ 

milli¬ 

milli¬ 

feet 

feet 

feet 


gauss 

gauss 

gauss 

gauss 

gauss 

gauss 

M-4 Medium tank 

450 

400 

290 

150 

180 

50 

5 

4 

3 

3-in. Gun carriage 

460 

310 

340 

200 

130 

100 

4 

8 

4 

British cruiser-tank 

320 

320 

280 

100 

100 

60 

2 

5 

2 

Half-track 

510 

600 

630 

200 

150 

180 

1 

4 

9 

Scout-car 

120 

90 

140 

70 

50 

40 

1 

2 

5 

6-ton 6x6 Truck 

170 

120 

130 

70 

70 

50 

2 

4 

4 

Command car 

150 

120 

70 

60 

40 

40 

1 

2 

1 

*4-ton 4x4 Truck 

130 

120 

100 

80 

50 

70 

0 

2 

1 

^-ton 4x4 Truck 

100 

80 

90 

60 

60 

70 

1 

2 

1 

37-mm Gun carriage 

140 

100 

120 

50 

50 

50 

2 

3 

4 

37-mm Gun carriage 

100 

110 

100 

60 

80 

60 

1 

2 

1 


what part of the vehicle passes over the mine, 
and the estimated distance from the vehicle at 
which firing may take place for mines having a 
sensitivity of 50 milligauss. 


detonation. It should be remembered that the 
fields would vary with the magnetic latitude, 
with the data of Table 1 applying in the middle 
northern latitudes. 



The vehicles tested may be roughly divided 
into two classes: track-laying vehicles and 
wheeled vehicles. In general, the records of the 
former are characterized by large and erratic 
values for all three components, while the fields 
of the latter are smaller and more uniform. 


Possibility of Degaussing 

A large part of the field of the tank in the 
middle latitudes appears to be due to magnetism 
induced by the earth's vertical field. This could 
be partially neutralized by an arrangement of 


CONFIDENTIAL 
















































MAGNETIC LAND MINES 


75 


coils which would produce a distribution of 
north poles along the bottom of the side armor 
of the tank. That part of the field due to the 
earth's horizontal field could be neutralized by 
a system of coils which would approximate the 
current distribution shown in Figure 7. Unless 
arrangements, preferably automatic, are made 
to make this current in these coils proportional 
to the cosine of the magnetic heading of the 
vehicle, the compensation could be off for some 
headings by as much as 160 milligauss. It might 
perhaps be necessary to provide another ad¬ 
justable current arrangement to neutralize 
magnetism induced in the tank by the earth’s 
horizontal field on easterly and westerly head¬ 
ings. The magnitude of this effect was not 
measured but it should be smaller than that 
obtained on northerly and southerly headings. 

It is probable that the presence of magnet¬ 
ically permeable material in the tank would 
enable a simple current distribution to produce 
a better fit to the field than would be expected 
from the simple treatment. This would simplify 
the problem. A rough estimate has been made 3c 
of the amount of copper and current needed 
to do the degaussing. For a current of 5 amp 
from the 24-v electric system of the tank, 
about 1 kg of copper, suitably arranged in coils, 
would be required. Most of the records show 
major irregularities, and the records under the 
tracks show irregularities extending a few feet 
at most. The latter “fine structure” could prob¬ 
ably be removed by local demagnetization or 
“deperming.” It is probably out of the question 
for any practical degaussing system to com¬ 
pensate for the vehicle’s magnetic field to such 
an extent that it would be incapable of actu¬ 
ating the most sensitive firing device possible. 

Conclusions 

The measurements on the magnetic fields of 
vehicles indicate that the use of magnetically 
operated land mines against vehicles is entirely 
feasible. The vertical component of the field is 
superior to the longitudinal and transverse 
components for firing a mine. To assure firing 
on the horizontal components of the field, a 
mine must respond to both positive and nega¬ 
tive fields, which would often result in firing 


when the vehicle was not directly over the mine. 
On the other hand, if the mine responds only 
to positive vertical fields, an appropriate choice 
of sensitivity would insure its firing only under 
the vehicle. Magnetically operated mines need 
not be touched by the tracks or wheels of 
vehicles to be fired, permitting an area to be 
adequately mined with fewer units. They could 
be so adjusted that they would be fired only by 
the heavier armored units, permitting the 
lighter reconnaissance vehicles to pass over 
them unharmed. 

Thorough degaussing and deperming of ve¬ 
hicles is out of the question if done simply 
enough to be practicable, although it could be 
done for mines of low sensitivity. One possibil¬ 
ity that merits serious consideration is that of 
using magnetically operated mines of low sen¬ 
sitivity together with simple degaussing of our 
own heavily armored vehicles. This would per¬ 
mit our own vehicles, both lightly and heavily 
armored, to pass unharmed over minefields for¬ 
bidden to armored vehicles of the enemy. 

3 3 MAGNETIC LAND MINES 

Since the measurements of the magnetic 
fields of vehicles described above had shown 
magnetically operated mines to be feasible, 
work was undertaken on the development of a 
magnetic firing device for land mines. Two 
entirely different devices were developed. 

3,3,1 Mechanical Magnetic Firing Device 

This magnetic firing device for land mines 
was developed at the Department of Terrestrial 
Magnetism, Carnegie Institution of Washing¬ 
ton. 2 It depends for its operation on the me¬ 
chanical motion of a sensitive magnet which 
takes place when the vertical component of the 
field changes. The construction is such that only 
rough leveling is required at the time of instal¬ 
lation, while gradual changes in level and 
normal changes in the magnetic field are auto¬ 
matically compensated for. 

The firing device consists of a magnet system 
suspended so that it is free to rotate about a 
horizontal axis when acted on by changes in the 
vertical component of the earth’s field, and a 


CONFIDENTIAL 



76 


MAGNETIC CHARACTERISTICS OF VEHICLES AND MAGNETIC LAND MINES 


vane which rotates with the magnet system for 
gradual changes in the field but which lags 
behind for rapid changes, causing an electric 
contact to be made. 

The magnet system consists of six Alnico 
magnets, 3/32 by 13/32 by 5/8 inches, mounted 
three each on two small tubular shafts as shown 
in Figure 8. A larger tube, which has an open 
portion to permit the insertion of the vane, 
serves as a hub connecting the two shafts. The 
vane is mounted on a very small steel shaft, 
the ends of which are seated in insulated pivot 
bearings in the ends of the larger tube so that 


around the magnet system. This is enclosed in 
a large tube about 2 in. in diameter and 6 in. 
long, as shown in Figure 9. This tube is nearly, 
but not quite, filled with oil and then sealed. 
The oil serves to damp the motion of the vane, 
making it unable to follow rapid motions of the 
magnet system. 

If the earth’s vertical field is suddenly in¬ 
creased, the south poles of the magnets move 
rapidly upward. The vane, however, lags behind 
and makes contact with the uninsulated arm, 
completing the electrical circuit about 0.1 sec¬ 
ond after the change of the field. The models 



Figure 8. Magnet system and supporting frame of firing device (magnets, 1; tubular 
shafts, 2; suspension wires, 3; tension springs, 4; plastic plates, 5; tubular hub, 6; vane, 
7; insulated arm, 8; contact arm, 9). 


the vane is free to rotate about the axis of the 
magnet system. The entire magnet system is 
suspended by phosphor bronze wires which ex¬ 
tend from flat springs through the tubular 
shafts to about the position of the middle mag¬ 
net of each group of three. Only one of the 
suspension wires is electrically connected to 
the magnet system, the other being electrically 
connected to the vane. Two rigid arms, one 
with an insulated tip, are attached to the tubu¬ 
lar hub. The vane is slightly unbalanced so that 
normally it rests against the insulated tip. 

A frame of brass and plastic holds the flat 
springs to which the suspension wires are at¬ 
tached and forms a squirrel-cage structure 


constructed were sensitive to changes of 25 
milligauss taking place in times less than 1 sec¬ 
ond. In other words, it will be actuated when 
a jeep is driven over it at a speed greater than 
2 or 3 mph. For use against armored vehicles, 
the device could be made less sensitive by in¬ 
creasing the separation of the contacts. 

Several possible variations in the use of the 
device present themselves. (1) The explosive 
charge need not be located in the same place as 
the firing device itself; e.g., it could be used to 
detonate a series of charges along a highway 
when a vehicle reaches one part of the highway. 
(2) The device could have remote control for 
arming and disarming, such as is used in harbor 


CONFIDENTIAL 



MAGNETIC LAND MINES 


77 


mines. (3) It is believed that the device can be 
made so rugged that it may be dropped from 
aircraft into areas behind the enemy’s lines. A 
great increase in ruggedness can be obtained 
by using a heavy oil and freezing it before the 
device is dropped. 

# A wide range of variations in sensitivity and 
time of response can be obtained, particularly 
for applying the device to marine use. The 
greater space and weight practicable for marine 
mines would permit much greater sensitivity 
and slower response, which would adapt the 
mine for use even against degaussed ships. 


be actuated by vehicles traveling from 1 or 2 to 
approximately 45 mph. 

The basic principle of operation is essentially 
the same as that of the automatic recording 
magnetometer described in Section 3.2.1. The 
difference is that the output voltage of the sec¬ 
ondary coil is used to trip a control tube which 
discharges a condenser through the firing cap, 
instead of being amplified and recorded on a 
meter. 

The detector consists of two tiny matched 
cores of a high-permeability steel alloy, the 
primary coils for which are connected in op- 



Figure 9. Firing device in tubular housing. 


Electronic Magnetic Firing Device 

This magnetic firing device was developed at 
the Gulf Research and Development Company 5 
and is an adaptation of the device developed 
there for use in submarine influence mines 
(discussed in Chapter 4). The firing device is 
complete and self-contained; it may be used as 
a separate unit or attached to a standard anti¬ 
tank mine case as shown in Figure 10. The 
transparent plastic cover illustrated was se¬ 
lected for demonstration purposes only. The 
unit is sensitive to an increase of about 50 
milligauss in the earth’s vertical field and will 


position. The coils are excited to saturation by 
a relaxation oscillator, and the output voltage 
of the common secondary coil is proportional 
to the component of the magnetic field parallel 
to the cores. b The output pulses from the 
secondary are rectified by a small vacuum tube 
operating as a grid-leak detector. The rectified 
voltage is applied to the grid of the second tube, 
which amplifies the changes if they occur at 
rates corresponding to a prescribed range of 
vehicular speeds. In this way the effects of slow 
drifts are eliminated. The amplified voltage is 
applied to the grid of a small thyratron tube, 
b Footnote a explains this relation. 


CONFIDENTIAL 






78 


MAGNETIC CHARACTERISTICS OF VEHICLES AND MAGNETIC LAND MINES 


causing it to fire when the voltage changes by 
an amount corresponding to a change in mag¬ 
netic field of 50 milligauss or more. The thyra- 



Figure 10. Electronic firing device mounted on 
standard anti-tank mine. 

tron tube discharges a condenser through the 
firing cap so that it is not necessary to have a 
battery of sufficient current capacity to fire 


stationary magnetic field through the coils so 
that the device will operate at a point near the 
center of the linear portion of the detector-tube 
characteristic curve. This makes it possible to 
adjust the device for satisfactory operation 
in any magnetic field normally encountered 
throughout the world. It can be done for a 
given zone of operations before planting, pro¬ 
vided the mines are not to be planted near 
large steel objects. Even so, the adjustment is 
easy to make in the field with the help of a 
simple test instrument. 

When the mine is planted, the firing circuit 
is disconnected. The clockwork timing relay is 
set to connect the circuit after a selected inter¬ 
val which is adjustable from 5 minutes to 1 
hour. An additional time delay is then provided 
by the charging time of the condenser. 

The units constructed apparently have good 
characteristics of sensitivity, response to vari¬ 
ous vehicle speeds, and low battery consump¬ 
tion. Considerable care was taken to select 
circuits which would give uniform results with¬ 
out critical matching of tubes and other com¬ 
ponents, and to give as nearly uniform results 


E 0 p 



Figure 11. Circuit diagram for electronic firing device. 


the cap directly. Figure 11 gives a circuit dia¬ 
gram of the complete device. 

A compensating magnet is used to adjust the 


as possible over the life of the batteries, but no 
attempt was made to design the unit for quan¬ 
tity production. 


CONFIDENTIAL 


























































Chapter 4 


CONTROL SYSTEM AND DETECTORS FOR 
SUBMARINE INFLUENCE MINES 

By F. L. Yost a 


41 INTRODUCTION 

S ubsurface mines (referred to in this chap¬ 
ter as “submarine mines” or merely 
“mines”) controlled from shore are widely used 
for harbor defense. A large number of mines 
are arranged in a pattern so that any vessel 
entering the harbor must come within the de¬ 
structive area of one or more mines. Adequate 
signaling to shore from the mines and control 
of the mines from shore are necessary in order 
to protect friendly vessels. 

At the time when the present work was 
undertaken it was usual practice to arrange 
mines in groups of nineteen, each connected to 
a distribution point by means of a current- 
carrying single-conductor cable. A single-con¬ 
ductor cable connected the distribution point 
to the shore installation. The mines installed 
by the Submarine Mine Depot, Fort Monroe, 
Virginia, were usually of the buoyant type, 
each so anchored by a cable (to a weight on 
the bottom of the ocean) that the mine was 
held a little below the surface of the water. 
The tilting of a mine by a vessel armed it by 
closing contacts that made it possible for the 
mine to be exploded either automatically or by 
manual operation at the shore station. 

The initial buoyant mine had the advantage 
of not requiring any power to be supplied to 
the mine to operate the detector. The original 
control system operated on a current margin 
basis and required maintenance of voltages 
and currents within rather close limits. The 
system operated on a step-by-step basis; that 
is, when one mine was contacted, the control 
system had to step through all the mines, in 
order, until it reached the proper one. Only 
after that indication had been noted and the 
control had been released could the apparatus 

a Technical Aide, Division 17, NDRC. 
b The control numbers for this project were CAC-1, 
OD-69, and OD-72. 


step on to another struck mine or around to 
its home position. Only one mine could be ex¬ 
ploded at a time. It was felt that a frequency- 
control system might be developed which would 
eliminate the close margins and would at the 
same time afford a more flexible control. As a 
result, a project was set up with the Union 
Switch and Signal Company 14 for studies and 
experimental investigations of circuits used to 
operate mines at the Submarine Mine Depot, 
for the purpose of recommending possible im¬ 
provements. 

Complete circuit diagrams and descriptions 
of operations were drawn up for five different 
systems, all applicable to the then standard 
contact mines. The fifth system was very versa¬ 
tile and could be operated automatically, semi- 
automatically, or manually. Therefore, the first 
four systems were eliminated from considera¬ 
tion, and it was decided to build a model of the 
fifth system for the control of two groups of 
nineteen mines each. When this work was sub¬ 
stantially completed, the possibility of influence- 
type mines replacing the contact-type was 
proposed. 

The Coast Artillery Corps had decided that 
the influence-type mine was more satisfactory 
than the contact-type. For example, greater 
sensitivity and a higher percentage of actuation 
could be expected. The possibility of using 
influence-type mines rather than contact-type 
introduced new problems for the control system 
(e.g., supply of power for the detector). It was 
felt that any system developed should provide 
for the control of influence mines. Since the 
system under construction was not well suited 
for this purpose, work on it was dropped im¬ 
mediately. Plans were drawn up for a new 
frequency-control system which could be used 
with influence mines. This work was coordi¬ 
nated with work on a project by the Gulf Re¬ 
search and Development Company 515 on a mag¬ 
netic detector and associated circuits for mines, 


CONFIDENTIAL 


79 



80 


CONTROL SYSTEM AND DETECTORS FOR SUBMARINE INFLUENCE MINES 


and with work on a project by the Massachu¬ 
setts Institute of Technology 16 23 on a sonic 
detector and associated circuits for mines. 

This report deals with the frequency-control 
system for influence mines and the magnetic 
and sonic detectors. 


42 MILITARY REQUIREMENTS 

The features required for a successful con¬ 
trol system are the following. 

1. The apparatus must perform reliably. 

2. The system must clearly and quickly indi¬ 
cate to the operator when a ship is within strik¬ 
ing distance. If a ship comes within striking 
distance of several mines simultaneously, it is 
highly desirable that they should indicate 
simultaneously. 

3. The warning should be both audible and 
visual. The audible signal may be a single-stroke 
gong to attract the operator’s attention, but 
the visual signal must remain on until answered 
by the operator. 

4. Any mine or group of mines must be capa¬ 
ble of being exploded from shore, whether or 
not the presence of a ship has been indicated. 

5. Detonation must never take place from 
any cause other than a deliberate act on shore. 

6. Some means of testing must be provided 
so that the condition of all parts of the ap¬ 
paratus (except the detonation mechanism) can 
be checked from time to time. 

7. If the mine mechanism requires power 
for its operation, the power must be supplied 
over the shore cable. 

8. There should be no equipment other than 
fuzes at the distribution point (i.e., at the point 
at which the individual cables from the mines 
in a group are connected to the single shore 
cable which services the group). 

The characteristics required for the magnetic 
detector are: (1) it should have a detection 
radius of 75 ft; (2) it must operate with, and 
as a complementary part of, the control system. 
In addition, it would be desirable if it were self- 
powered. 

The specifications of the sonic detector are: 
(1) it should have a cone of influence such that 
the surface area to which actuations of the 


device are limited is a circle 140 ft in diameter, 
70 ft above the mine; (2) as a countermining 
measure, the device should function only in the 
area described and at sound pressures less than 
500 bars. 


4 3 SUMMARY OF DEVELOPMENT 

4,3,1 Control System 

All the control requirements are met by a 
frequency-control system in which each mine 
signals to shore by means of a distinctive fre¬ 
quency when a boat is near. The control of the 
mine, both for testing and for firing, is pro¬ 
vided by other frequencies (distinctive for each 
mine), and the power is supplied over the shore 
cables. The control system, as finally developed, 
will operate with any type of mine, the only 
requirements being that the mine contain an 
oscillator and that the output of the oscillator 
be changed by a substantial percentage when 
a detection is made, irrespective of the mode of 
detection. 

432 Magnetic Detector 

One of the detectors developed for use with 
this control system was an electronic magnet¬ 
ometer. This detector is not self-powered, but 
suitable provision is made for supplying power 
from shore through the control system. Many 
tests of the actual effectiveness of this detector 
have been made by plotting the courses of vari¬ 
ous classes of vessels (all degaussed) over a 
single line of five mines spaced 150 ft apart 
and at an average depth of about 75 ft. In these 
tests the mine-control apparatus gave actual 
firing indications for an average of 90 per cent 
of the passages over this single-line minefield. 
The percentage of indications varied from 
about 40 per cent for small vessels from 75 to 
150 ft long to 99 per cent for large vessels 
(larger than destroyers) , 14 The actual effective¬ 
ness of a complete minefield, such as might be 
used for harbor protection, would be much 
higher than these figures, as a more complete 
net of mines than the single line of five would 


CONFIDENTIAL 



SUMMARY OF DEVELOPMENT 


81 


have to be traversed. Also, the effectiveness 
would be increased by the operator's judgment 
of the position of the ship relative to mines 
which give merely warning signals but not a 
second (i.e., firing) signal. 

In the above tests the mines were on sea 
bottom, as would be the case in actual practice. 
When a detecting device is employed, a larger 
charge is used than would be needed for a 
contact-type mine and the mine is placed on 
sea bottom as a precaution against counter¬ 
measures. 


4 ‘ 3 * 3 Sonic Detector 

Sonic detectors were developed for use with 
the control system. Complete circuits for an 
echo-type detector were reported; and two units 
were delivered to the Mine Depot and placed 
under continual test. This device worked and 
was being improved. Such improvements hardly 
seemed worth while, however, because of the 
necessary complexity of the device when used 
for controlled mines, and the fact that the 
device tended to fire on wakes of ships. Hence, 
further work on this type of device was discon¬ 
tinued. Since it seemed urgent that some sort 
of acoustic device be designed, preferably a 
simple one, work was then begun on a 15-kc 
listening-type detector to operate with the 
shore-control equipment. The device was in¬ 
stalled at Fort Monroe, where its operation 
proved very satisfactory. Reports on the first 
two months of tests showed that not one boat 
passed within the destructive range of the mine 
(approximately 150 ft) without giving a firing 
indication. 23a There were a number of times 
when the boats set the light off too soon. How¬ 
ever, a small amount of premature firing was 
considered tolerable in order to keep the device 
extremely simple. The percentage of actuations 
within the destructive range of the mine was 
very high. For example, during one eleven-day 
period thirty-five courses were plotted of boats 
of all types (battleships to small patrol boats) 
which passed over the unit. There were no 
misses: 77 per cent of the actuations were 
within 150 ft and 23 per cent were over 150 ft 
away. 


4 * 3 ' 4 Complete System 

In the final development, the unit of offshore 
equipment consists of thirteen mines, each con¬ 
nected by an individual cable and fuze to a 
common distribution box connected by a single 
cable to shore controls which may be as far 
away as eight miles. All energy transmission 
for functioning of the unit—which includes sig¬ 
naling from any mine to shore and testing or 
firing of any mine from shore—takes place over 
the single cable. Complete, independent control 
to and from each of thirteen mines over the 
single cable is maintained by energy transmis¬ 
sion at a series of different frequencies and 
discrimination by means of suitable filters. 

Each mine can detect the presence of a vessel 
as a result of a magnetic or sonic disturbance. 
It can send to shore a signal frequency specific 
to itself, indicating the presence of a vessel in 
its vicinity. It can accept from shore the signal 
frequency necessary for testing the condition 
of its detector and the signal frequencies nec¬ 
essary for its detonation. 

The shore apparatus has provision for per¬ 
forming the following functions for a number 
of offshore units. 4 It supplies power to the mine 
apparatus. For each shore cable and group of 
thirteen attached mines it provides selective 
and indicating circuits which clearly indicate 
any change in the sonic or magnetic condition 
at each mine and at all mines. It supplies thir¬ 
teen test frequencies and has a switching ar¬ 
rangement so that any one or group of these 
frequencies can be applied to any shore cable 
or group of shore cables. It provides the firing 
frequency and a switching arrangement so that 
this frequency can be applied to any shore cable 
or group of shore cables. The control system is 
flexible enough so that various types of detec¬ 
tors can be used with it; the indication receiv¬ 
ers are the only part of the apparatus requiring 
modification with change of detector. 

The entire mine development may be sum¬ 
marized by enumerating the advantages of the 
system, most of which were new at the date of 
its development. The system operates on in¬ 
fluence mines on sea bottom, thus leaving the 
channel free of obstructions. It is stable, de¬ 
pendable, long lived, and gives reliable indica- 


CONFIDENTIAL 



82 


CONTROL SYSTEM AND DETECTORS FOR SUBMARINE INFLUENCE MINES 


tions of ships. It has a simple, visible signal 
and an auxiliary audible signal, the two per¬ 
mitting judgment of proper firing time. It 
shows indications of all mines at all times on 
a very simple indication panel. Any one or any 
group of mines can be fired at any time, but 
only by deliberate action of the shore operator. 
The operation of the system can be checked and 
its sensitivity adjusted from shore. The system 
operates on a single conductor to each distri¬ 
bution point, has unit construction permitting 
expansion to cover needs of different installa¬ 
tions, and uses individual plug-in type electric 
units to facilitate replacement and repair in 
case of trouble. 

Original requirement No. 8 is not satisfied 
because it is necessary to place at the distribu¬ 
tion point a 440- to 220-v transformer and some 
filter elements. 

Early in 1943 officers of the Coast Artillery 
Board witnessed demonstrations of the control 
equipment, and it was decided that the devel¬ 
opment showed sufficient promise to warrant 
the building of two production samples for field 
tests. The development work terminated in 
June 1943 with the awarding of a contract by 
the Ordnance Department for the building of 
two five-group equipments comprising the con¬ 
trol equipment necessary to operate five groups 
of mines and the detecting equipment for these 
mines. In each five-group installation, three 
groups were to be of the magnetic type, one 
group of the sonic type (i.e., the listening de¬ 
vice), and one group a sonic detector of a type 
developed by the Naval Ordnance Laboratory 
(with suitable modification by the Submarine 
Mine Depot). 


44 DESCRIPTION AND TECHNICAL 
INFORMATION 

4 41 Control System with Magnetic Detector 

A functional diagram of shore and mine ap¬ 
paratus used with the magnetic detector is 
given in Figure 1. The frequencies used and the 
assignment of different frequency ranges to 
specific purposes are indicated in Figure 2. The 
general arrangement of the mine, of the mag¬ 


netic detector, and of the associated electric 
equipment is shown in Figure 3. The magnetic 
detector itself is placed in a long nonmagnetic 
housing extending above the main body of the 
mine. The complete assembly of the mine ap¬ 
paratus, as shown in Figure 4, contains seven 
vacuum tubes and has approximately the same 
number and kind of parts as a seven-tube radio 
receiver. To ensure long life and stability, the 
circuits are all carefully designed so that the 
loads of the vacuum tubes are much less than 
usual. The minimum tube life should be of the 
order of two years. 

The magnetic detector is entirely electric in 
operation and has no moving parts. The de¬ 
tector coil, shown in Figure 5, is essentially a 
transformer, the secondary of which surrounds 
two primaries which are parallel to each other 
and which are each wound around a thin strip 
of high permeability material (Mu-metal). An 
exciter oscillator in each mine excites the pri¬ 
maries oppositely and with sufficient current to 
saturate the cores. The wiring and arrange¬ 
ment are such that, in the absence of any com¬ 
ponent whatsoever of magnetic field along the 
primary cores, simultaneous and opposite ener¬ 
gizing of the primaries results in zero sec¬ 
ondary voltage. If a component of magnetic 
field is imposed along the cores, it adds to the 
exciting field of one core and subtracts from 
the opposing one of the other core at every 
instant of an energizing cycle. The secondary 
voltages induced by the primary coils are there¬ 
fore not equal and opposite, and they cancel 
each other only partially. The net result is a 
complex secondary voltage which for a detector 
placed in the earth’s field has a minimum value 
for zero ambient field and increases with ap¬ 
plied ambient field component. 

A detector tube rectifies and amplifies the 
secondary voltage, delivering continuously a 
d-c voltage. As shown in Figure 6, the effective 
range through which the secondary voltage 
changes approximately linearly with variations 
of field imposed upon the primary cores extends 
from about 0.1 to 0.9 gauss. As the magnitude 
of the earth’s field is in the range from about 
0.3 to 0.8 gauss, and the changes which may be 
induced by a passing ship are a fraction of the 
earth’s field, it is evident that the range avail- 


CONFIDENTIAL 



DESCRIPTION AND TECHNICAL INFORMATION 


83 


able is well suited to this application. To have 
a relatively uniform response to change in mag¬ 
netic field over as wide a range as possible, the 
detector is biased with a small compensating 
magnet to operate about a mid-point corre¬ 
sponding to a field of approximately 0.5 gauss. 

Since the detector output is measured by the 
amplitude of the secondary voltage, the exciter- 
oscillator was developed to deliver a constant 
output in spite of normal variations in the d-c 
plate supply. The output from the magnetic- 


shore apparatus operates a test and firing relay 
in the corresponding mine. The relay is desig¬ 
nated testing and firing because it performs two 
functions. (1) It energizes a small magnetic 
field about the Mu-metal cores, simulating an 
outside magnetic disturbance of fixed magni¬ 
tude. The response on shore to this test field is 
a measure of the overall operation and sensi¬ 
tivity of the entire apparatus. (2) It conditions 
(or “selects’’) the mine to be detonated upon 
simultaneous application of a firing frequency. 


I-1 



L 


SHORE EQUIPMENT 


J 


Figure 1. Functional diagram of shore and mine apparatus. 


detector secondary goes to a detector-amplifier 
tube which modulates the output of an oscil¬ 
lator signaling back to shore. The oscillator is 
set to operate at one of the thirteen indication 
frequencies in the range from 2 to 10 kc, the 
particular frequency being specific to the mine 
in which the oscillator is installed. The oscillator 
output is coupled to the mine cable through a 
filter-amplifier which insures that no false sig¬ 
nals can be impressed on the cable. 

A selection and test frequency supplied from 
shore at the will of the operator may be applied 
to the detector. There are thirteen test frequen¬ 
cies between 250 c and 1,350 c, each assigned 
to a particular mine and receivable by that 
mine through a tuned filter. Impression of one 
of these frequencies on the mine cable from the 


If a 200-c firing frequency (same for all 
mines) is supplied to a mine at the same time 
that the selection and test frequency is supplied, 
the mine will explode. A firing filter in each 
mine is tuned to accept the 200-c frequency. 
Thus, to explode a given mine, it is necessary 
that the test frequency specific to that mine 
be applied at the same time that the firing fre¬ 
quency common for all mines is applied. The 
achievement of a satisfactorily safe and relia¬ 
ble circuit which could not be actuated by any 
conceivable combination of accidental circum¬ 
stances was most troublesome and is one of the 
most important accomplishments of the entire 
development. 

In addition to the specific functions which 
have been mentioned, the mine apparatus in- 


CONFIDENTIAL 


















































84 


CONTROL SYSTEM AND DETECTORS FOR SUBMARINE INFLUENCE MINES 


eludes an adjustment which corrects for any 
permanent change in magnetic field, such as 
might be caused by the tilting of the mine 
resulting from the explosion of a near-by mine. 
This correction process is fast enough to take 
care of natural or diurnal changes of the earth’s 
magnetic field but is too slow to affect the sen¬ 


ators and one firing oscillator supplies all units 
in the installation. Any oscillator is rendered 
operative by closing the appropriate oscillator 
switch on the control panel. The cable to which 
the chosen frequency is supplied is determined 
by closing the proper switch for testing detec¬ 
tors. The firing oscillator may be connected to 


All frequencies for 13 mines carried between shore and submarine Junction box by a 
single-conductor oable and between Junction box end mines by 13 single-conductor cables. 


© © 



a u 

<r) O fl 1 

a xi o © 

©•HO 

Firing 

Seleotion and Test 

a _ ♦» a 
•h g o £ 

PY*equency 

Frequencies 

© h a ** 

© Q ■»-> M 
.o a 

Supplied 

Supplied from shore at will. 

3 © © -r* 

♦» ♦» O +> 

from ehore 

of shore operator. Each 

i— i -■* a 
rl O hrl 

at will of 

mine is responsive to one 

r-* ► a 3 • 

• _ a tso © 

shore opera¬ 

frequency only. A relay 

b * 3 P O 

tor. Will 

controlled by this response 

O •<* © -H 

v, o a 

fire a mine 

is used to apply a standard 

♦» a ♦» 

«-» 

for which 

test signal to the detector 

l-t © CO 

CX -O ♦> 3 

seleotion 

for checking signal from 

a © o h 3 

3 1H B O -H 

© C >• > 

. S' P 

frequency 

mine and also to set firing 

ia supplied 

cirouit to be responsive to 

h O. %■* O T3 

© 3 D W fl 

© co a CM 

B. 2 +» a 

at the same 
time. 

firing frequency. 

• +>©»« 



a o -a © 



|I!» 



Ut O *» X> V, 



440 V 

1 

50V 


Mill f 1 V | M M r 


t. a 
© © 

§1 

*** 

H 

•a r-t 


* s 

a S © 
© 3 ♦> 

3 ® o 
o ♦> u 

♦>33 

o o. 

2 u a 


2 50 V 
D|C 


Indication Frequencies 


Supplied continuously by 
mine apparatus, eeoh mine 
supplying a different 
frequency. The amplitude 
of the signal from each 
mine is modified by the 
response of the mine 
deteotor to the presence 
of a ship near the mine. 


IV 


I I I I I I I I I I 


ITT 


Excitation 

Frequency 

Exciter 

oscillator 

in mine 

vhloh 

supplies 

excitation 

ourrent for 

primary of 

electronic 

magnetometer, 


T“r 

o o 
o o 
o o 


FREQUENCY 

Figure 2. Frequency diagram for magnetic-influence submarine mine system. 


sitivity of the detector to changes caused by 
any normally moving ship. 

The shore equipment consists of a power sup¬ 
ply for the mines, selection and firing oscillators, 
indication receivers, the necessary filters, and 
a control panel. 

For each cable there is a mine power-supply 
unit which supplies 250 v direct current and 
440 v alternating current for the plates and 
filaments respectively of the ninety-one tubes 
in each group of thirteen mines. Power-supply 
voltages are effectively separated from each 
other and from the signal and selection fre¬ 
quencies by condensers and chokes in the mine 
apparatus. One set of thirteen selection oscil- 


any cable by closing the appropriate pull-type 
firing switch on the control panel. 

A signal frequency from a mine oscillator, 
as modulated by the detector, comes in over the 
shore cable, is admitted by a filter to the ap¬ 
propriate indication receiver, is amplified and 
operates an indication light on the control 
panel. The amplifier also operates a single¬ 
stroke bell whenever any light goes on. Signals 
coming in on another cable operate similar 
apparatus and produce similar responses in 
another signal receiver and indicator unit. 

A schematic diagram of a harbor-defense 
network involving ten units is shown in Fig¬ 
ure 7. The general appearance of the main 


CONFIDENTIAL 





























DESCRIPTION AND TECHNICAL INFORMATION 


85 


indication and control panel for ten units is 
shown in Figure 8; and this figure also shows 
the panel indications for the hypothetical target 
of Figure 7. 

Associated with each mine are two lights on 
the control panel which show the condition of 
the mine continuously. When the first light 
comes on, it indicates that a ship is approaching 
the mine; when the second light comes on, that 
the ship is close to the mine. With the activa¬ 
tion of each light, a single-stroke electric bell 


MAGNETIC-DETECTOR 

COIL 



Figure 3. Arrangement of mine, magnetic de¬ 
tector, and associated electric equipment. 


rings once. The first light serves as a warning 
signal; the second, as a firing signal. The prog¬ 
ress of a ship following a specific course through 
a minefield is shown on the indication panel 
by the lighting of lamps associated with mines 
near enough that course to be influenced. An 
operator having such an indication and having 
also means to fire any one or any combination 
of mines could effectively control the fate of 
the vessel attempting to traverse the minefield. 
All signal lights on any row, corresponding to 
one shore cable and its thirteen attached mines, 
may be extinguished and all apparatus reset to 
operating condition by moving the switch at the 
right of that row to the “cancer’ position. 

To test a given mine, the operator closes the 


“unit selection” switch at the bottom of the 
column containing the mine (this energizes 
the oscillator supplying the frequency to that 
mine) and moves the switch at the right of the 
row containing the mine to the “test” position 
(this connects the proper mine cable to the 
selected oscillator). 

If all apparatus functions, this test produces 
a signal back to the shore apparatus, which is 
indicated by the lighting of one of the lamps 
associated with the selected mine. The mine 
apparatus also contains a tilt indicator which, 
when the mine is seriously off level, modifies 
the mine circuit in such a way that a two-way 
impulse is sent back to the shore apparatus 
when the test signal is applied, so that both 
lamps are lighted instead of only one. 

The mine is fired by simultaneous operation 
of the “unit selection” switch in the column 
containing the mine and of the special pull-type 
firing switch. This applies power to the oscil¬ 
lator operating at the test frequency of the 
mine so selected, applies power to the firing- 
frequency oscillator, and connects the firing and 
test frequencies to the mine cable. The simul¬ 
taneous application of the test and firing fre¬ 
quencies operates two relays which fire the 
detonator and explode the mine. 

The entire operation of the mine-control sys¬ 
tem described here depends upon the response 
of the mine detector to the magnetic field of a 
ship. The signal amplification available is suffi¬ 
cient so that if it is all used (by adjusting a 
variable gain control on the shore amplifier) 
the indication lights will respond to the noise 
level of natural or artificial changes in the 
magnetic field. The maximum sensitivity which 
may be used, therefore, depends on the mag¬ 
netic environment of the location. In remote 
installations, far from artificial magnetic fields, 
a sensitivity approaching 0.1 milligauss for 
signal operations might be possible. None of 
the degaussing or deperming treatments of 
ships have reduced the fields so much that they 
are not appreciably stronger than the minimum 
at which the mine detector can be made effec¬ 
tive. In fact, the degaussing treatment tends 
to be an advantage because it effectively re¬ 
stricts the response to the immediate neighbor¬ 
hood of the ship. 


CONFIDENTIAL 





































84 


CONTROL SYSTEM AND DETECTORS FOR SUBMARINE INFLUENCE MINES 


eludes an adjustment which corrects for any 
permanent change in magnetic field, such as 
might be caused by the tilting of the mine 
resulting from the explosion of a near-by mine. 
This correction process is fast enough to take 
care of natural or diurnal changes of the earth’s 
magnetic field but is too slow to affect the sen¬ 


ators and one firing oscillator supplies all units 
in the installation. Any oscillator is rendered 
operative by closing the appropriate oscillator 
switch on the control panel. The cable to which 
the chosen frequency is supplied is determined 
by closing the proper switch for testing detec¬ 
tors. The firing oscillator may be connected to 


frequencies for 13 mines carried between shore and submarine junction box by a 
single-conductor cable and between Junction box and mines by 13 single-conductor cables. 


il 

H 
•O rH 


2 8 
CU<M 


+» o. • 
a a a 
© 3 


3 • O 
O ♦> U 

© T* 

<H O 

o o. 

2 u § 


2 50 V 
D|C 


© © 



Son 1 
a » S 3 

Firing 

Selection end Test 

a . 4» « 

■h 0 o u 

Frequency 

Frequencies 

© U 3 ** 

Supplied 

Supplied from shore at will. 

3 « © 

+» +» a +» 

from shore 

of shore operator. Each 

H r( © 
rl O hH 

at will of 

mine is responsive to one 

r-» ► a 3 • 

• 0 00 n 

shore opera¬ 

frequency only. A relay 

O £> © © 
h 3 3 a 

tor. Kill 

controlled by this response 

O ■>* CO -rt 

o a 

fire a mine 

is used to apply a standard 

♦» a +> 

>» « -H «H 

r-t © S3 

Cl, tJ b ♦» 3 

for which 

test signal to the detector 

selection 

for cheoking signal from 

O. © © r-l •© 

3 -H a O T-t 
OH R ► 1. 

0.0 Ti 

frequency 

mine and also to set firing 

is supplied 

circuit to be responsive to 

u o.u o -o 
©sown 

at the same 

firing frequency. 

Filament pow 
olrcuits. S 
to auto-tran 
box and at 2 
formers in i 

time. 


440 V 

1 

50V 

/- 2 0 V -^ 

i i i i i it i i i i i i 


Indication Frequencies 


Supplied continuously by 
mine apparatus, eeoh mine 
supplying a different 
frequency. The amplitude 
of the signal from each 
mine is modified by the 
response of the mine 
deteotor to the presence 
of a ship near the mine. 


I I I 


- IV- 

I I I I I 


I I 


Excitation 

Frequency 

Exciter 
oscillator 
in mine 
vhioh 
supplies 
excitation 
current for 
primary of 
electronic 
magnetoms ter, 


1 T 


FREQUENCY 

Figure 2. Frequency diagram for magnetic-influence submarine mine system. 


sitivity of the detector to changes caused by 
any normally moving ship. 

The shore equipment consists of a power sup¬ 
ply for the mines, selection and firing oscillators, 
indication receivers, the necessary filters, and 
a control panel. 

For each cable there is a mine power-supply 
unit which supplies 250 v direct current and 
440 v alternating current for the plates and 
filaments respectively of the ninety-one tubes 
in each group of thirteen mines. Power-supply 
voltages are effectively separated from each 
other and from the signal and selection fre¬ 
quencies by condensers and chokes in the mine 
apparatus. One set of thirteen selection oscil- 


any cable by closing the appropriate pull-type 
firing switch on the control panel. 

A signal frequency from a mine oscillator, 
as modulated by the detector, comes in over the 
shore cable, is admitted by a filter to the ap¬ 
propriate indication receiver, is amplified and 
operates an indication light on the control 
panel. The amplifier also operates a single¬ 
stroke bell whenever any light goes on. Signals 
coming in on another cable operate similar 
apparatus and produce similar responses in 
another signal receiver and indicator unit. 

A schematic diagram of a harbor-defense 
network involving ten units is shown in Fig¬ 
ure 7. The general appearance of the main 


CONFIDENTIAL 



























DESCRIPTION AND TECHNICAL INFORMATION 


85 


indication and control panel for ten units is 
shown in Figure 8; and this figure also shows 
the panel indications for the hypothetical target 
of Figure 7. 

Associated with each mine are two lights on 
the control panel which show the condition of 
the mine continuously. When the first light 
comes on, it indicates that a ship is approaching 
the mine; when the second light comes on, that 
the ship is close to the mine. With the activa¬ 
tion of each light, a single-stroke electric bell 


MAGNETIC-DETECTOR 

COIL 



Figure 3. Arrangement of mine, magnetic de¬ 
tector, and associated electric equipment. 


rings once. The first light serves as a warning 
signal; the second, as a firing signal. The prog¬ 
ress of a ship following a specific course through 
a minefield is shown on the indication panel 
by the lighting of lamps associated with mines 
near enough that course to be influenced. An 
operator having such an indication and having 
also means to fire any one or any combination 
of mines could effectively control the fate of 
the vessel attempting to traverse the minefield. 
All signal lights on any row, corresponding to 
one shore cable and its thirteen attached mines, 
may be extinguished and all apparatus reset to 
operating condition by moving the switch at the 
right of that row to the “cancel” position. 

To test a given mine, the operator closes the 


“unit selection” switch at the bottom of the 
column containing the mine (this energizes 
the oscillator supplying the frequency to that 
mine) and moves the switch at the right of the 
row containing the mine to the “test” position 
(this connects the proper mine cable to the 
selected oscillator). 

If all apparatus functions, this test produces 
a signal back to the shore apparatus, which is 
indicated by the lighting of one of the lamps 
associated with the selected mine. The mine 
apparatus also contains a tilt indicator which, 
when the mine is seriously off level, modifies 
the mine circuit in such a way that a two-way 
impulse is sent back to the shore apparatus 
when the test signal is applied, so that both 
lamps are lighted instead of only one. 

The mine is fired by simultaneous operation 
of the “unit selection” switch in the column 
containing the mine and of the special pull-type 
firing switch. This applies power to the oscil¬ 
lator operating at the test frequency of the 
mine so selected, applies power to the firing- 
frequency oscillator, and connects the firing and 
test frequencies to the mine cable. The simul¬ 
taneous application of the test and firing fre¬ 
quencies operates two relays which fire the 
detonator and explode the mine. 

The entire operation of the mine-control sys¬ 
tem described here depends upon the response 
of the mine detector to the magnetic field of a 
ship. The signal amplification available is suffi¬ 
cient so that if it is all used (by adjusting a 
variable gain control on the shore amplifier) 
the indication lights will respond to the noise 
level of natural or artificial changes in the 
magnetic field. The maximum sensitivity which 
may be used, therefore, depends on the mag¬ 
netic environment of the location. In remote 
installations, far from artificial magnetic fields, 
a sensitivity approaching 0.1 milligauss for 
signal operations might be possible. None of 
the degaussing or deperming treatments of 
ships have reduced the fields so much that they 
are not appreciably stronger than the minimum 
at which the mine detector can be made effec¬ 
tive. In fact, the degaussing treatment tends 
to be an advantage because it effectively re¬ 
stricts the response to the immediate neighbor¬ 
hood of the ship. 


CONFIDENTIAL 





































86 


CONTROL SYSTEM AND DETECTORS FOR SUBMARINE INFLUENCE MINES 


The pattern of the magnetic field below a 
ship is highly variable, depending on the ship, 
its magnetic history, and its heading with re¬ 
spect to the magnetic meridian and the mag¬ 
netic latitude. For a degaussed ship the pattern 
may be quite irregular, with one or more posi- 


of either a positive or negative area of the 
disturbance field passes over the mine. This 
change will result in the lighting of one of the 
indication lamps. As the ship moves on, the in¬ 
tensity of the magnetic field at the mine will 
continue to change in the same direction until 



Figure 4. Complete assembly of mine apparatus and magnetic-detector coils. 


tive and negative areas which, at depths of the 
order of 100 ft, have magnetic intensities of 
the order of 1 to 10 milligauss. Whatever the 
details of the magnetic pattern may be, the 
first change strong enough to affect the mag¬ 
netic detectors will occur when the outer part 


the central part of that particular disturbance 
area passes the mine, whereupon the field will 
begin to change in the opposite direction. This 
reversal results in the lighting of the second 
of the indication lamps; in general, this occurs 
when the ship is relatively close to a mine. 


CONFIDENTIAL 








DESCRIPTION AND TECHNICAL INFORMATION 


87 


Thus the first light serves as a warning that 
a ship is approaching the mine, and the second 
that it is close, and the mine should be fired 
then if the ship is to be destroyed. 

In early test apparatus, a pen recorder was 
used to record the magnetic-field-intensity indi- 

GROUNDED ELECTROSTATIC 



cations transmitted to shore by the equipment 
for certain types of vessels. Some of these are 
shown in Figure 9. The time scale (running 
from left to right in all cases) is the same for 
all the records, and so, roughly, is the magnetic- 
field scale. The dashed lines indicate coincidence 
of the bow and the stern of a ship with the 
line of sight from the control point to the posi¬ 
tion of the detector. The courses of the ships 


were not exactly normal to the line of sight and 
the ships might have been on either side of the 
mine and possibly out of the danger zone. The 
crosses on the records show the time of flashing 
of the indication lamps. 

It is fallacious to base the firing of the mines 
on a predetermined field strength because mines 
will then blow up far ahead of strong targets. 
Use of time delays is also objectionable because, 
if they provide appreciable improvement of fir¬ 
ing action on large, slow targets, they will 
permit fast ships to slip through. Firing at the 
point where the magnetic field reverses is some¬ 
what better because the ship’s field never re¬ 
verses at great distances. However, a ship’s 
signature may fail to reverse, especially if it 
is not degaussed. Accordingly, the firing-indi¬ 
cator arrangement used is so designed that 
reversals are created where they do not actually 
exist. 



FIELD IN GAUSS 

Figure 6. Magnetic-detector characteristic. 

For a schematic diagram of the complete final 
magnetic mine unit (except for the firing cir¬ 
cuit) see the Gulf Research final report. 14 For 
diagrams and pictures of the mine detonator 
circuit and the shore apparatus see the final re¬ 
port 4 of the Union Switch and Signal Company. 


CONFIDENTIAL 


















































88 


CONTROL SYSTEM AND DETECTORS FOR SUBMARINE INFLUENCE MINES 


442 Control System with Sonic Detector 

Paralleling the development of a magnetic 
detector, a 15-kc listening-type detector unit 
was developed for use with the control cir¬ 
cuits. 22 * 23 The military requirements placed on 
the performance of the sonic detector by the 
Submarine Mine Depot have already been listed 
(Section 4.2). 

The first of these requirements (specifying 
the cone of influence) could not be realized in 


specifically requested, were suggested as very 
desirable. Irrespective of the acoustic firing 
device in the mine, it should be possible to check 
from shore the overall sensitivity of the electric 
circuit in the mine. Changes in the plate power- 
supply voltage of ±5 v should not affect the 
operation of the device, nor should transients 
in the line. The device should be so designed 
that any adjustment in the circuit that may 
become necessary because of aging may be 
made on shore. The circuit should contain a 


JUNCTION BOX CABLES TO SHORE 



/ 


/ 

/ TARGETS COURSE 


Figure 7. Shore-controlled minefield. 


the device designed for the Depot; nor could it 
be realized for any other acoustic firing device 
designed for continuous operation on a con¬ 
trolled mine system and suitable for immediate 
production. A compromise was made in order 
that a number of units could be built without 
delay. In the listening unit a directional hydro¬ 
phone is used; it is this feature which gives the 
mine its cone of influence and satisfies the first 
requirement within the limits consistent with 
the necessary simplicity for a controlled mine 
unit. The second requirement (actuation at 
sound pressures less than 500 bars) is easily 
satisfied. 

A number of other features, though not 


minimum number of tubes, and it is a great 
advantage to use the same type of tube through¬ 
out the entire circuit. All these useful features 
are incorporated in the final design of the sonic 
detector. 

The sonic development may be summarized 
as follows. The unit has but one microphone, 
and it may therefore be classed as a one-channel 
system. There is only one amplifier whose gain 
needs checking, and this may easily be done 
from shore. With a one-channel system, the 
decrease in amplifier sensitivity with age may 
be compensated for by means of a calibrated 
adjustment on the shore control equipment— 
the same control used for varying from shore 


CONFIDENTIAL 















DESCRIPTION AND TECHNICAL INFORMATION 


89 


the sensitivity of the magnetic detector. The 
electrical features of the particular system used 
which contribute to its simplicity and stability 
are: (1) use of simple conventional circuits; 
(2) use of a single type tube (taken from the 
Army-Navy preferred list) for all tube posi¬ 
tions; (3) ability to use all tubes which con- 


substantially complete and independent systems 
being built on separate chassis panels. Each 
subassembly is designed for mass-production 
methods and individual testing, servicing, and 
interchanging. 

The block diagram for the apparatus at the 
mine is shown in Figure 10. Circuit diagrams 


FIRST LIGHT ON __ 
FOR EACH MINE 
SHOWS APPROACH 


FIRST MINE IN GROUP 
SHOWING A SECOND 
LIGHT INDICATES 
TARGET IN FIRING 
POSITION 


SWITCHES FOR 
SELECTING MIN 
FOR FIRING OR 
TESTING 


METERS TO CHECK MINE POWER SUPPLY 


( AC 
iAMP5 


( A-C \ 

(volts! 


( D-C > 
lAMPSj 


( D-C > 

(volts; 


i «© 2 


o o o o 
O o o o 


o o o o o o 
o o o o o o 


o o 
o o 


cm 


n •©» 8 


o o o o 
o o o o 


o o o o o o 
o o o o o o 


o o 
o o 


CD 


in «£• 8 


o o o o 
o o o o 


o o o o o o 
o o o o o o 


o o ™ 


12 <£> o 


O O o o 
o o o o 


o o o o*-*- *• 
o o o o o o 


o o . ■. 

o o ™ 




o o o o 
o o o o 


o o * * * * 

o o o # o 


o o 
o o 


CD 


21 ©S 


o O o •. 
0 0 0 6 


* * m o o 


*r o' o o 


o o 
o o 


cm 


IH <©0 


** * * 
O * * * 


-fc O O o o o 
0 0 0 0-0 0 


o o 
o o 


CCD 


nn ^ § 


o o o o 
o o o o 


o o o o o o 
o o o o o o 


o o 
o o 


cm 


IX © 


o o o o 
o o o o 


o o o o o o 
o o o o o o 


o o 
o o 


cm 


X |@> o 


o o o o 
o o o o 


o o o o o o 
o o o o o o 


o o 
o o 


CCD 



II 01 

2 3 4 5 


UNIT 


6 7 8 9 10 II 
SELECTION 


12 13 


I 0FF<O 


ON 




SWITCHES FOR TESTING 
DETECTORS AND 
CANCELLING INDICATION 
LIGHTS 


Figure 8. Control panel for ten units. 


form to the normal commercial tolerances with¬ 
out any special selection; (4) insensitivity to 
small changes in plate-supply voltage; (5) use 
of a time-delay circuit, which prevents spurious 
responses of the device to accidental individual 
transients. 

Mechanically, the device makes use of the 
unit type of construction, parts which form 


and pictures of the equipment are given in the 
contractor’s report. 7 

For a number of reasons it was desirable to 
listen at a frequency of 15 kc. A hydrophone 
(microphone) commercially available from the 
Brush Development Company has a beam pat¬ 
tern of about 45 degrees at 15 kc, as shown 
in Figure 11. This hydrophone (type AX-26-1) 


CONFIDENTIAL 
























92 


CONTROL SYSTEM AND DETECTORS FOR SUBMARINE INFLUENCE MINES 


May and found to be far less erratic, but not 
sensitive enough. Methods for increasing the 
sensitivity were suggested. Part of this im¬ 
provement was to be made in the mine equip¬ 
ment and part in the indication receiver used 
on shore. Cathode-ray tubes were suggested for 
the shore equipment to indicate the behavior 
of the mine equipment. Listening devices for 
indicating the presence of a ship were intro¬ 
duced. It was during this time that the officers 
at Fort Monroe decided that fully automatic 
operation was not worth while and chose fully 

bap.le I □ —' T e 
HYDROPHONE 


180* 



Figure 11. Directional characteristic of Brush 
Model AX-26-1 Hydrophone mounted in baffle 
shown. (Testing distance—30 in.) 

manual equipment for controlling and firing 
the mine. 

During June and July a simplified manual- 
control system was built for two groups with 
thirteen mines in each group. The new indica¬ 
tion receivers were provided with a sensitivity 
adjustment, and the cathode-ray tubes were 
added to indicate the behavior of the detectors. 
Four of the magnetic detectors were provided 
with acoustic listening-type devices with the 
hope that both magnetic and acoustic indica¬ 
tions could be obtained from the same mine. 

Tests on this equipment were conducted 
during the latter part of July. During the 
period from August to October 1942 additional 
magnetic firing devices and indication receivers 
were constructed. 

The tests conducted at Fort Monroe showed 
the merits of the control system and at the same 
time uncovered many minor faults which were 


gradually eliminated. As the behavior of the 
detector unit improved, the desirability of the 
cathode-ray tubes disappeared; early in 1943, 
it was decided to abandon them altogether and 
to depend entirely on indication lamps. A num¬ 
ber of tests and demonstrations of this equip¬ 
ment were made, including the exploding of 
detonators. Ships entering the harbor were 
observed when they crossed the minefield where 
these experimental mines were located. Much 
information was obtained, showing that the 
magnetic detector operating with this fre¬ 
quency-control system gave very good results 
on all types of vessels, especially on the smaller 
ones. 


4 5 2 Magnetic Detector 

The present magnetic units are the result of 
a great deal of similar developmental work. The 
first two experimental samples of a magnetic 
detecting and firing device were tested at Fort 
Monroe in the summer of 1941. 7 This device 
sent to shore a signal of constant amplitude, 
variations in the frequency of which indicated 
magnetic changes at the mine. The sensitivity 
of the device was more than adequate, but it 
had many shortcomings. The exciting oscillator 
lacked stability; although apparently satisfac¬ 
tory when new, its behavior became progres¬ 
sively worse with aging. Firing indications 
were effective only about 50 per cent of the 
time. Part of the trouble was due to high sensi¬ 
tivity plus the noise level, part to blocking of 
the amplifier circuit, which would upset the 
detector for many minutes after passage of a 
strong target. Another limitation of the device 
was that it required a separate conductor for 
each mine. 

The variations in firing indications observed 
with these detectors were attributed to the 
magnetic patterns of different ships as well as 
to shortcomings of the original apparatus. 
Accordingly, data on the magnetic fields of 
ships were obtained and efforts made to pro¬ 
duce improved detectors. A new unit, Set C, 
was delivered to the Submarine Mine Depot in 
November 1941. 14a It required a heavy set of 
batteries for operation of the detector and 


CONFIDENTIAL 











HISTORY 


93 


seemed obsolete shortly after completion, in 
view of the possibility of greater flexibility 
offered by the frequency-control system which 
was being developed separately. In December 
1941 a conference was held to coordinate work 
on the shore-control system and on the mag¬ 
netic detectors. As a result, each was changed 
to suit the other; the changes improved both 
parts of the new combination. The control 
apparatus was actually simplified and improved 
in the process of incorporating a power source 
for operating the magnetic detectors; at the 
same time, it was made suitable for other types 
of detectors using vacuum-tube equipment. 

By May 1942, 26 detectors, called Test Set 
D, were delivered in major part for use with 
the frequency-control apparatus. 8 ’ 9 The de¬ 
tector was powered entirely by direct current 
from shore. This was a big step forward. How¬ 
ever, in spite of improvements in the oscillator, 
the detector was still subject to gas-tube noises 
because gas-tube regulators for plate-supply 
voltage were used in each mine. These proved 
unsatisfactory because their voltages would 
occasionally jump a fraction of a volt, which 
was sufficient to give false indications. The 
detector oscillator was prone to fluctuate at 
times due to the cold-cathode gas-tube oscillator 
used. It was therefore decided to abandon cold- 
cathode gas tubes. This was made possible in 
succeeding models by progress in the fre¬ 
quency-control equipment which allowed ample 
60-c power to be transmitted to the detectors 
to supply heater power for a-c tubes. 

Test Set E was the first unit to use a-c tubes 
throughout. 10 A 2050 exciting oscillator tube 
was used, with great improvement over the 
cold-cathode tube. A vacuum-tube surge sup¬ 
pressor replaced the noisy gas-tube regulators, 
and a limiter circuit was added to improve 
firing characteristics and recovery time from 
disturbances. Two of these units were made, 
and one was delivered to the Submarine Mine 
Depot in July 1942. A similar type of unit, 
Test Set F, included provision for acoustic and 
magnetic reception from one unit, using one 
carrier frequency to convey both signals. 10 The 
magnetic signal provided the usual slow modu¬ 
lation of the carrier, but the acoustic device 
provided modulation in faster rates in the 


neighborhood of 240 c. The two rates of modu¬ 
lation were separated by dual shore receivers 
so that separate indications were obtained. 
Four of these units were delivered in July 1942 
for tests at Fort Monroe. The combined 
acoustic-magnetic detectors operated satisfac¬ 
torily, except that very strong magnetic signals 
blocked the acoustic signals and very strong 
acoustic signals shifted the points at which 
magnetic indications occurred. 

To cut down the peak current in the 2050 
oscillator, a revised circuit was developed. 
Units with this oscillator were designated Test 
Set E2. 11 Then this oscillator was replaced by 
a vacuum-tube oscillator, which was very much 
steadier, and a few other changes were made 
at the same time. The new type was called Set 
E2'. 11 Fifteen units built as type E2 and one 
built as type E were converted to type E2' for 
delivery to the Submarine Mine Depot late in 
November 1942. 

Set E2' was superior to all previous types, 
but still had a source of occasional noise. At 
that time the set was redesigned to eliminate all 
the troubles that had been found and to incor¬ 
porate Army-Navy preferred tubes. This new 
design was designated Test Set E3. 13 This was 
the final experimental design of which six were 
built and put on test in March 1943. 

Through the entire development the principal 
difficulty was with noise. It is difficult to make 
sensitive circuits that will be completely quiet 
day after day in operation. Since the start of 
the development, circuits could be built and 
were built that would operate apparently with 
a very satisfactory noise level; but occasional 
bursts of noise occurred when they were run 
day after day on recorders, when tubes were 
replaced, or when circuits were duplicated in 
number. Gas tubes, the chief source of these 
troubles, were eliminated entirely from the 
detectors, and every other precaution was taken 
to achieve a high degree of continuous, quiet 
operation. 


4 * 5,3 Sonic Detector 

When the development of the acoustic detec¬ 
tor was undertaken, four different methods of 


CONFIDENTIAL 



94 


CONTROL SYSTEM AND DETECTORS FOR SUBMARINE INFLUENCE MINES 


detection were considered: (1) acoustic range 
finding, (2) listening-type control unit, (3) 
beam-type control unit, and (4) echo-type unit. 
The listening-type control unit has several out¬ 
standing advantages, such as simplicity, low 
power drain, ease of adaption to control sys¬ 
tem, and ease of combination with the mag¬ 
netic firing device. In addition, it has the 
advantage that the only countermeasure 
against a minefield of such units would be to 
cross it on “hand” control. The echo-type unit 
had the advantage of being one of the few 
practical devices that did not depend on the 
output of sound from the ship. Hence, a sub¬ 
marine could not slip by it. 

The first acoustic detector developed in this 
work was an echo-type unit. 21 In this develop¬ 
ment an acoustic signal of high-frequency 
sound is produced by a Rochelle-salt transducer 
(a device which acts either as a microphone 
or loudspeaker). The signal consists of a suc¬ 
cession of pulses, each pulse consisting of 
high-frequency sound, approximately one milli¬ 
second in duration, occurring at a rate of about 
ten per second. After the transducer sends out 
a pulse it acts as a microphone. It hears an 
echo from the surface of the water and any 
other nearby reflecting object. However, the 
equipment is so designed that it is responsive 
only during a time interval which begins sev¬ 
eral milliseconds after the initial pulse and 
which ends shortly before the surface echo 
arrives back at the transducer. In order to be 
detected, an object must therefore come closer 
to the mine than a distance equal to slightly 
less than the depth of the water. 

Two circuits were developed. The first was 
designed to operate on battery-type vacuum 
tubes; the second, on heater-type tubes, operat¬ 
ing on alternating current made available at 
the mine. Two mine-control units of this type 
were put on continuous test at Fort Monroe. 

The fundamental design of either circuit is 
the same. An oscillator tube drives the crystal 
transducer which produces the acoustic pulse. 
The transducer then acts as a microphone con¬ 
nected to the input of an amplifier. If an echo 
is received, it is amplified and detected. How¬ 
ever, the detector tube is supplied with plate 
voltage only during the period in which the 


device is to be operative. During the operative 
period, if an echo is received, it raises the 
potential at the plate of the detector tube and 
also at the control electrode of a cold-cathode 
gas trigger tube which follows the detector. 
If a strong echo is received, the potential at 
the plate of the detector will become great 
enough to discharge the gas tube. This dis¬ 
charge supplies a large pulse to an integrating 
filter. If this happens a number of times in 
succession, the device sends an indication back 
to shore. Thus, isolated spurious echoes and 
line transients will not produce a shore indi¬ 
cation. 

Three types of transducers were considered 
when the project was started: (1) magneto¬ 
striction, (2) quartz crystal, and (3) Rochelle 
salt crystal. Of the three, the Rochelle salt 
crystal has the greatest sensitivity; as units 
of this type were available from the Naval 
Research Laboratory, they were used. 

Two types of apparatus were tried for exci¬ 
tation of the crystal transducer. The first em¬ 
ployed a Strobotron. The breakdown of the 
gas tube excited the crystal transducer. This 
circuit was economical in that the cold-cathode 
tube required no power for heating the filament. 
However, the characteristics of different tubes 
were not very uniform, and the operation of 
the circuit was not very stable; hence it was 
decided to try a Hartley-type oscillator employ¬ 
ing a low-drain filament-type tube. An oscil¬ 
lator of the self-quenching type was used. Since 
the total power input to the Hartley-type 
oscillator was less than that for the Strobotron 
type, and since the former was found to be more 
stable and to give as much output as the latter, 
the Hartley-type oscillator was used in the 
final design of the circuit. 

This detector did work and was being im¬ 
proved; but, for reasons already mentioned, 
it was decided to drop further work on it 
and to concentrate on the development of a 
listening-type device. 

Some exploratory work had been done on lis¬ 
tening devices at 250 c and at 100,000 c. 21 When 
work was started on the design of a listening- 
type device, the Submarine Mine Depot 
submitted the desired characteristics already 
mentioned. In order to approximate these with- 


CONFIDENTIAL 



HISTORY 


95 


out too much developmental work, a listening 
frequency of 15 kc was chosen. A listening-type 
acoustic firing device was developed. This was 
installed for tests which proved its operation 


satisfactory. Subsequently the Submarine Mine 
Depot requested six more units for further 
testing. The Depot also decided to place a con¬ 
tract for a production quantity of this device. 


CONFIDENTIAL 



Chapter 5 

GUN RANGING AND LOCATING SYSTEMS 

By Frank Woodbridge Constant 


51 INTRODUCTION—SUMMARY OF 

RESULTS 

T his report contains a complete technical 
description of the work performed by the 
Division of Physical War Research at Duke 
University under its contract with OSRD. The 
body of this report is addressed to readers of 
the type represented by the technical men of 
NDRC and the technical officers and civilians 
of the Army and Navy. 

Section 5.1 describes the organization of the 
project and summarizes its results. Section 5.2 
presents the general background of the subject 
in order that the relation of each phase of the 
work to the whole may be better understood; 
it also includes a brief description of methods 
and equipment in acoustic gun ranging which 
were standard at the time the project was 
initiated. Section 5.3 covers the study and 
improvement of standard methods and equip¬ 
ment, and Section 5.4 deals with the develop¬ 
ment of new methods and equipment. These 
two sections describe the majority of the work 
undertaken, considered from two angles: (1) 
obtaining the necessary data and (2) handling 
the data after it has been obtained. Under each, 
first the methods and then the equipment re¬ 
quired are discussed. Section 5.5 explains the 
main part of the physical research—a funda¬ 
mental study of sound transmission through the 
atmosphere, with emphasis on those micro- 
meteorological factors which produce fluctua¬ 
tions and irregularities in sound propagation. 
Section 5.6 discusses accessory projects and 
work of a general consulting nature. Section 
5.7 gives the present status of the subject as 
a whole. 

As each particular project is taken up in 
detail, a brief historical description of the 
earlier work is followed by a technical descrip¬ 
tion of the final form of the apparatus de¬ 
veloped or the conclusions reached. How well 
the Division succeeded in meeting specific 


military requirements is stated in each case. 

More detailed descriptions of any phase of 
the work can be found by referring to the 
Bibliography at the end of the report. 


5,1,1 Purpose and Organization 1 

Purpose. This project as a whole was con¬ 
cerned with the investigation of the detection 
and location of enemy guns, mainly by sound. 
Particular emphasis was placed on the location 
of enemy artillery. 

The military importance of quickly locating 
enemy guns of all kinds made very desirable a 
fresh, coordinated attack on the problem. The 
fluidity of the military situation in World War 
II, in contrast to the relatively static front of 
World War I, added to the importance of the 
problem. The urgency of improved gun ranging 
was expressed repeatedly by the Field Artillery 
Board of the Army Ground Forces during the 
course of the work. 

The project originated with Section C5 (later 
Section 17.3) of NDRC, whose chief was 
Harvey Fletcher. V. 0. Knudsen, a member of 
this Section, was assigned as Supervisor of 
the project, which was later designated as No. 
17.3-3. Fletcher and Knudsen, with W. S. 
Gorton, Technical Aide to the Section, visited 
the Field Artillery School at Fort Sill, Okla¬ 
homa, and the University of Oklahoma at 
Norman, Oklahoma, and then the Field Artil¬ 
lery Board at Fort Bragg, North Carolina, 
together with the University of North Carolina, 
at Chapel Hill, and Duke University, at Dur¬ 
ham, North Carolina. NDRC wanted to locate 
the work near one of the above military reser¬ 
vations and at a university where the necessary 
facilities would be available. As Duke Univer¬ 
sity seemed to meet these requirements best 
and expressed willingness to undertake the 
work, it was awarded an OSRD contract. 11 The 
a Contract No. OEMsr-734. 


96 


CONFIDENTIAL 



INTRODUCTION—SUMMARY OF RESULTS 


97 


Division of Physical War Research was set up 
at Duke University to operate under this con¬ 
tract, which was later supported by the Army 
Control No. SOS-13 and Navy Control No. 
MC-100. 

The activities of the Division began at Duke 
University officially on September 15, 1942, 
under the directorship of J. P. Maxfield. Space 
was allocated in the Physics Building. The 
personnel grew steadily: on January 1, 1943, 
the scientific and technical staff numbered 
thirteen; on August 1, 1943, it numbered 
twenty-three and later totaled thirty. 

In order to familiarize the Division of 
Physical War Research with the methods and 
systems of gun ranging by sound and with 
the problems to be solved, members of the 
Division were invited to attend conferences 
with military personnel. Existing methods of 
acoustic gun ranging were discussed and their 
shortcomings pointed out. No specific require¬ 
ments were given at the start of the work. The 
Division was asked to consider the subject as 
a whole and to investigate possible improve¬ 
ments in existing methods and equipment, as 
well as the feasibility of introducing new meth¬ 
ods or systems. In general the two main re¬ 
quirements to be kept in mind were increased 
accuracy and increased speed of operation. 

Organization. The work was organized and 
proceeded along four main lines: (1) the 
development and improvement of existing 
methods and equipment, (2) the study and 
development of new methods and equipment, 
(3) physical research on the fundamental 
principles of sound propagation and sound 
ranging, and (4) general consulting work for 
the Armed Forces. 

The physical research group was concerned 
with the fundamental principles of sound 
ranging. Its work thus included a study of 
sound-ranging methods in use or under investi¬ 
gation, as well as a study of the best methods 
of handling the data obtained from any of the 
systems being developed. This group also served 
the Division in another manner: its members, 
by keeping familiar with the work of the 
Division as a whole, were available for consul¬ 
tation in regard to the fundamental principles 
of the various projects being developed by the 


engineering groups. Instances arose where a 
problem begun under the physical research 
group reached such an advanced stage that it 
became a development project and was classi¬ 
fied as such. As a consequence, some results of 
immediate use were produced, new methods 
and equipment were developed for future use, 
and a background of fundamental information 
necessary for long-term development work was 
obtained. 

As work along the above lines progressed, 
it became possible for the Field Artillery Board 
to request the Division to push specific projects 
that seemed especially promising or of funda¬ 
mental importance. Then, as particular pieces 
of equipment were developed, the Field Artil¬ 
lery Board and (through an extension of the 
original contract) the Marine Corps Equip¬ 
ment Board, Quantico, Virginia, assisted in 
subjecting the equipment to a series of field 
tests. As a result of these tests the Armed 
Forces recommended specific alterations and 
improvements, so that the equipment in its 
final form would meet their military require¬ 
ments. In this connection it should be men¬ 
tioned that the Field Artillery Board, the Field 
Artillery School, and the Marine Corps Equip¬ 
ment Board were always most cooperative and 
helpful. Relations with these groups were 
mutually cordial and friendly. 

The Armed Forces gave many indications not 
only of the importance they attached to the 
work, but also of their satisfaction with its 
progress and their continuing confidence in the 
Division. For example, in the case of the Dodar, 
an earlier model (D-2) was tested by the Field 
Artillery Board. It added a few military re¬ 
quirements to the functions of the original 
and then recommended the adoption of the 
Dodar even before the new model had been 
designed and tested. After the new model had 
been completed, orders were received for over 
90 instruments of the new type, mostly for 
combat use. Arrangements were made for the 
manufacture of these instruments on a “crash 
procurement” order. 

In general, the Division was granted, when 
necessary, the highest priorities for procuring 
equipment. Finished instruments were in sev¬ 
eral cases flown to the fighting fronts. On one 


CONFIDENTIAL 



98 


GUN RANGING AND LOCATING SYSTEMS 


occasion Mr. Maxfield was flown by Army 
bomber to England to consult with British 
scientists and military personnel. Without such 
support and cooperation from the Armed 
Forces it would not have been possible for the 
Division to have accomplished nearly as much 
as it did in the way of supplying the Armed 
Forces with new instruments and information. 

Further evidence of the success of the Divi¬ 
sion in meeting military requirements is to be 
found in reports received from the fighting 
fronts which describe the behavior of certain 
instruments developed by the Division and 
subjected to actual combat use. 

5,1,2 Standard Methods and Equipment 
in Sound Ranging 

Obtaining the Data 

Methods. A study of the straight base and 
comparison with other types of bases showed 
that the standard straight base adopted in 
World War II was the most advantageous for 
the type of fluid warfare generally encountered. 

Equipment. (1) The modified T-21-B micro¬ 
phone. An acoustic coupler (MX-346/PN 
adapter) was developed for the T-21-B con¬ 
denser microphone employed in the standard 
gun-ranging equipment (GR-3-C). This coupler 
extends the frequency response of the T-21-B 
microphone from 25 to 60 c, thereby improving 
the character of the records obtained with it 
by giving a sharper initial break, permitting 
some identification of gun sizes, and making 
recognition of the ballistic wave much easier. 
The modified microphone is also less susceptible 
to moisture. Modification parts were turned 
over to the Field Artillery Board and to the 
Marine Corps Equipment Board and were later 
flown to the fighting fronts. As a result of this 
work the Field Artillery Board included a 
similar extension of frequency range in the 
military requirements of the T-23-T5 hot wire 
microphone. 

2. The dry paper recorder. An investigation 
was made of the possibility of replacing the 
standard photographic method of recording 
with one employing a wax paper or a paper of 
the Teledeltos type. This was found possible, 


and a model using wax paper was developed. 
Elimination of photographic recording makes 
unnecessary the use of the chemicals and supply 
of water called for in a developing process. 
This is a great advantage in either hot, dry 
climates or in very cold climates. When it was 
found that a similar instrument, based on 
identical principles, was being developed under 
a Signal Corps contract, efforts were turned 
in other directions. 

3. The binaural outpost. A binaural outpost 
listening device designated Binop was devel¬ 
oped and demonstrated. Such a device offers 
the possibility of eliminating the outpost 
observer in the standard gun-ranging method. 

Handling the Data 

Methods. (1) Standard plotting and methods 
of weighting. An investigation was made of the 
possibility of reducing errors in location by 
improving the methods of evaluating the data. 
It was found that intersections of asymptotes 
resulting from sub-bases one sub-base length 
apart, i.e., alternate asymptotes, are most re¬ 
liable. The relative reliability of other intersec¬ 
tions was also evaluated. 

2. The analytical method of computing. A 
straight computation method was developed for 
determining the location of an enemy gun from 
the data obtained by the standard method. In 
this method the coordinates of the gun’s posi¬ 
tion are computed with formulas and the help 
of prepared tables. This method has the 
advantage of eliminating the plotting board 
normally employed in the standard method. 

3. The nomographic method of computing. 
Nomograms which use standard sound-ranging 
data and give both azimuth and range in a 
single setting were developed. Nomograms were 
computed for the cases of the 2- and the 4- 
sound-second base with all microphones opera¬ 
tive, and for the case of the 4-sound-second 
base with an inner microphone inoperative, 
two nomograms being necessary to take care 
of all possible situations in this last case. 

4. The ballistic-burst method. A method was 
developed for locating a gun from the record of 
the shell-burst and ballistic waves only. Such 
a method is useful when meteorological or other 
conditions are such that it is difficult or impos- 


CONFIDENTIAL 



INTRODUCTION—SUMMARY OF RESULTS 


99 


sible to identify the muzzle blast of any 
particular gun. This method may also be em¬ 
ployed to check the position determined from 
the gun wave when the shell-burst, ballistic, 
and gun waves are all recorded. 

The first step in this method permits the 
determination of the line of flight of a projectile 
from a record of its ballistic and shell-burst 
waves. The Field Artillery Board has approved 
this step and the Field Artillery School has 
issued an Appendix to Field Manual FM 6-120 
covering its use in the field. 

The second step in this method permits 
determination of a range term. The range 
calculated by this method was generally found 
to have an error from 2 to 5 per cent of the 
total actual range, due to variations in the 
muzzle velocity of the shell. These variations 
may be due to faulty powder charge or to wear 
on the tube of the gun. 

5. Study of meteorological corrections. A 
comparison was made of several methods of 
correcting for the effect of meteorological con¬ 
ditions on the estimated position of the sound 
source to be located. Application of the stand¬ 
ard meteorological correction method adopted 
by the U. S. Field Artillery for a straight base 
was found statistically to improve the results 
considerably, although the effectiveness varied 
considerably in individual cases. Results, based 
on a rather small amount of data obtained 
from actual artillery fire, indicated more elabo¬ 
rate methods to be little, if at all, superior 
to the standard Field Artillery method. 

Equipment. (1) Trace-reading templates. A 
transparent template was developed to assist 
in reading from the record on the photographic 
tape the corresponding time of arrival of a 
sound-wave impulse at a given microphone. 
This template makes it possible for enlisted 
personnel to read traces with a much improved 
accuracy since personal judgment as to the 
point of break is eliminated. Over 800 copies 
of this template were furnished the Field 
Artillery Board, and some were flown to the 
fighting fronts. 

2. Nomogram and accessories. Satisfactory 
prints on a semiopaque plastic of the computed 
nomogram were manufactured. A mounting 
device for effecting the required settings and 


readings in the field was constructed, permit¬ 
ting rapid and accurate use of the nomogram. 

3. Artillery plotting grids. The Field Artil¬ 
lery Board expressed a need for plotting grids 
having greater dimensional stability than that 
provided by the paper sheets then in use. Previ¬ 
ous experience with nomograms made it pos¬ 
sible to satisfy this need. Several grids printed 
on Lucite, and special indicators which can be 
attached to the grids by means of vacuum 
cups were produced, as well as other acces¬ 
sories. These plastic grids provide greater 
stability and accuracy. 

4. Ballistic-burst templates and tables. In 
connection with the ballistic-burst method men¬ 
tioned above, 100 sets of templates and the 
necessary time interval and range tables for 
obtaining the range term were manufactured 
and delivered to the Field Artillery Board for 
shipment overseas. These templates applied to 
the German 170-mm K18 and 210-mm Mrs 18 
guns. In addition, templates and tables were 
prepared for the American 155-mm M-l gun; 
these are to be incorporated into the Field 
Artillery School instruction course and manual 
covering the ballistic-burst method. 

5,1,3 New Methods and Equipment in 
Sound Ranging 

Obtaining the Data 

Methods. (1) The seismic method. A study 
was made of the possibility of detecting and 
ranging artillery by means of surface seismic 
waves. Previous work by the Signal Corps 
indicated the impracticability of using deep¬ 
traveling refracted and reflected seismic waves. 
The use of purely surface earth-borne waves 
was investigated with no practical results. 

2. The doppler effect method. An investiga¬ 
tion was made of the possibility of developing 
a method of sound ranging in which use is 
made of the doppler effect. This was also found 
to be impracticable. 

3. The multiple-short-base method. An in¬ 
vestigation was made of the possibility of 
developing a method of sound ranging employ¬ 
ing several short, two-microphone bases, suit¬ 
ably coordinated. Such a method was found to 


CONFIDENTIAL 



100 


GUN RANGING AND LOCATING SYSTEMS 


possess definite advantages to contribute to the 
mobility and speed of operation of the sound¬ 
ranging system. 

Equipment. (1) The Dodar. The development 
of new, lightweight, sound-ranging instruments 
for use in the multiple-short-base method was 
carried to completion. This equipment, which 
has been designated Dodar (Detection of Direc¬ 
tion and Range), includes as its principal ele¬ 
ment an electronic time interval measuring 
device. In operation, one Dodar unit yields a 
direction line to the sound source; when three 
units are used, a triangle of intersections is 
obtained from which the location of the sound 
source is obtained. Field tests indicated that 
enemy artillery could be located at ranges up 
to 8,000 yd (and in some instances up to 12,000 
yd), within an accuracy of 200 yd, and that 
a properly trained team could determine a 
gun location within one minute after reception 
of the sound signal. 

Twenty-five units of the first experimental 
model of the D-2 Time Interval Dodar were 
manufactured, at the request of the Marine 
Corps Equipment Board. Of these, twenty units 
went to the Marine Corps for trial under 
combat conditions, and one unit went to the 
British. An improved model, known as D-3 
(U. S. Signal Corps AN/PNS-1), was next 
developed at the request of the Field Artillery 
Board. One hundred units of this model were 
manufactured. The Dodar was placed in opera¬ 
tion by the Marine Corps during several 
campaigns; reports on the I wo Jima campaign 
indicate it was effective under battle conditions. 
Technical manuals covering the Dodar have 
been issued by the Marine Corps and the Army. 

2. The lightweight crystal microphone. As an 
important adjunct of a complete Dodar system, 
a new ultralightweight microphone for sound 
ranging was developed. This microphone em¬ 
ploys a crystal element capable of withstanding 
temperatures exceeding those likely to be en¬ 
countered in the field. It weighs about 4 lb 
and is considerably less bulky than the present 
T-21-B condenser microphone. Provision is 
made for quickly altering the frequency- 
response characteristics to permit the micro¬ 
phone to be used either with Dodar or with 
the standard sound-ranging recorders. The 


smaller size reduces the time and labor of 
installation. 

Handling the Data 

Systems of Coordination. Consideration was 
given to the best method of coordination to 
use when employing the Dodar system. It was 
concluded that this could best be determined 
by actual combat experience, and the decision 
was left to the Marine Corps. 

Probability of Gun-Location Errors. A 
theoretical method was worked out of evaluat¬ 
ing the probability of locating a target within 
200 yd when two direction lines with their 
angular errors and the spacing between their 
origins are known. The calculated results were 
put in graphical form. Actual errors observed 
during field tests with Dodars agreed well with 
those calculated. 

Reduction of Errors Due to Meteorology and 
Terrain. During the experimental tests of the 
Dodar sound-ranging system, effects due to 
meteorological and terrain factors were ob¬ 
served. Methods were sought to reduce these 
errors. A simplified form of the standard 
meteorological correction used by the Army 
Field Artillery Observation Battalion was 
adopted for reducing steady wind and tempera¬ 
ture effects. Attention was called to the effect 
of fluctuations in meteorological conditions (the 
“met”) and the need for further study of this 
subject, particularly in connection with short 
sub-bases. Terrain effects were found to be 
minimized by proper location of the micro¬ 
phones. 

514 Fundamental Study of Sound Trans¬ 
mission through the Atmosphere 

The physical research was concerned chiefly 
with a fundamental study of sound transmis¬ 
sion through the atmosphere, particularly 
along a boundary such as the ground. It was 
realized that the major inaccuracy in acoustic 
gun ranging is caused by the effects of 
meteorology and terrain on the sound during 
its transmission from the source to the detect¬ 
ing equipment. Therefore, the possibility of 
reducing errors of this type was continually 
kept in mind, first through an empirical investi- 


CONFIDENTIAL 



LOCATION OF ENEMY EQUIPMENT BY SOUND 


101 


gation of the possible relationships existing 
between the errors and such meteorological 
measurements as could be made, and second 
through a basic, long-term physical study, both 
experimental and theoretical, of sound trans¬ 
mission through the atmosphere. 

Investigation showed that meteorological 
errors fall into two classes: (1) those due to 
overall and slowly changing conditions (the 
“macromet”) and (2) those due to meteoro¬ 
logical factors which vary rapidly and in a 
random manner (the “micromet”). Particu¬ 
lar attention was given to the latter, and 
special micrometeorological equipment was de¬ 
signed. Both steady, single-frequency sources 
and impulse sources were used in a statistical 
study of the relationship between the ampli¬ 
tude of a sound wave and its time of arrival 
under varying conditions; certain interesting 
correlations were obtained. 

The effect of terrain was also investigated. 
Although this work yielded no results of 
immediate applicability to gun ranging, it 
indicated the possibilities of high-altitude 
listening for enemy troop movements, etc. 
Furthermore, a theoretical hypothesis of the 
propagation of sound along a boundary was 
developed which checks data at higher alti¬ 
tudes, where existing theory and experiment 
agree, but which is also in closer agreement 
with data at lower altitudes and grazing inci¬ 
dence than are existing theories. 

5,1,5 Accessory Projects 

Binaural Listening. In connection with the 
development of the binaural outpost, a small- 
size binaural listening system for detecting 
enemy equipment was developed. This system 
gives some information as to the direction from 
which noises are emanating. It may be con¬ 
sidered as a highly portable anti-infiltration 
warning device. 

Analysis of Field Records. Gun-ranging film 
records and data sent from the fighting front 
were analyzed for the Army Ground Forces. 
In several cases the analysis yielded quite 
different diagnoses from those which were 
obtained by inspection in the field. As a re¬ 
sult, specific recommendations were made 


about obtaining and analyzing combat data. 

Proposed Method of Sound Ranging Elimi¬ 
nating “Met” Corrections. A new method of 
sound ranging employing a two-dimensional 
microphone array was proposed and developed 
analytically. This method was shown to have 
application: (1) as a practical field method of 
sound ranging without the use of meteoro¬ 
logical corrections, (2) as a research method 
for investigating the effective major meteoro¬ 
logical structure for sound ranging, and (3) 
as a research method for investigating the 
effect of fluctuations in the meteorological con¬ 
ditions upon the accuracy of sound ranging. 

Sphinx. A preliminary attempt, terminated 
by the ending of World War II, was made to 
detect and locate caves by sonic means. Field 
experiments in which an M-l rifle was fired 
40 ft in front of a 3-ft cubical box indicated 
that it was possible to detect reverberations 
set up in the box 50 to 100 ft in front of the 
opening to the box. Subsequent tests on full- 
size caves with commonly available explosive 
sound sources did not yield a positive result, 
but indicated that sound sources with much 
higher energy content in a frequency range 
below 50 c were required to excite the rever¬ 
beration and resonant frequencies of such 
caves sufficiently to be detectable. 

General Consulting. Considerable consulting 
work on specific problems was carried out for 
the Armed Forces throughout the life of the 
project. Every effort was made to render such 
service as useful as possible. In a sense the 
whole project could be viewed in this light. In 
all phases of the work, close liaison with the 
Armed Forces was maintained. One valuable 
by-product was the education of military per¬ 
sonnel in the principles and operation of their 
sound-ranging equipment, enabling them to put 
it to more effective use. The liaison officers for 
the Armed Forces indicated that this alone made 
the project well worth while, even if no new 
equipment had been developed. 

5 2 LOCATION OF ENEMY EQUIPMENT 
BY SOUND 

The various projects undertaken by the Divi¬ 
sion of Physical War Research were all related 


CONFIDENTIAL 



102 


GUN RANGING AND LOCATING SYSTEMS 


and covered all aspects of acoustic gun ranging. 
In order to understand the purpose of each 
phase of the work and its relation to the whole, 
the particular problems faced and the results 
achieved, it is necessary to be familiar with the 
general background of gun ranging. 

5 * 21 The Standard Method 26 

Under the above heading will be described 
the standard method and equipment employed 
by the U. S. Field Artillery Observation Bat¬ 
talions before and at the time of our entry into 
World War II. This method received its main 
development in World War I; since then certain 


in any other position, the arrival times at the 
points of observation will be different. This 
difference increases as the source location 
moves away from the perpendicular bisector, 
and thus provides a measure of its angular 
displacement from this line. If two microphones 
are placed some distance apart and the differ¬ 
ence in arrival time of a sound at each micro¬ 
phone is recorded, the direction of a ray which 
passes very close to the origin of the sound 
may be determined. Other combinations of two 
microphones will provide similar rays, and 
from their intersections the source of sound 
may be located. 

Sound Base. In practice, a sound wave is 



Figure 1. Typical sound-ranging installation. 


modifications, mainly in equipment, have been 
introduced to improve the results. 

Basic Theory. A typical sound-ranging instal¬ 
lation is shown in Figure 1. The discharge of 
a gun or burst of a shell causes a sound disturb¬ 
ance, or vibration of the air, lasting for only 
a fraction of a second. The impulse so produced 
is propagated through the air in all directions 
at the same speed as any other sound, 360 to 
380 yd per second at average air temperatures. 
In still air, the sound will arrive at two given 
points at the same time if their distances from 
the source are equal—that is, if the source of 
the sound lies on the perpendicular bisector of 
the line connecting the two points. For a source 


detected by an array of four to six microphones, 
normally spaced at equal intervals (700 to 2,000 
or more yd) along a straight line or, under 
certain conditions, along the arc of a circle. 
However, they may be spaced at unequal inter¬ 
vals along a straight or broken line. Such an 
array is termed a sound-ranging base or sound 
base. A straight-line segment connecting a pair 
of adjacent microphones constitutes a sub-base. 
Normally, sound-ranging installations are accu¬ 
rately surveyed in. 

Recording. Each microphone is connected by 
a wire or radio circuit to the recording set 
located at the sound central. The sound impulse 
received at each microphone is recorded by this 


CONFIDENTIAL 










LOCATION OF ENEMY EQUIPMENT BY SOUND 


103 


equipment on a moving paper tape; these 
recorded impulses are called breaks. In front of 
the sound base, at distances of from 1,000 
to 2,000 yd, one or two outpost observers ( OP 1 
and OP . 2 ) are stationed. Either observer, upon 
hearing the sound of a gun or shell burst, must 
actuate the sound-ranging apparatus in time 
to record the sound. 

Oscillograms. The oscillogram is a paper tape 
upon which a time scale (1/100-second inter¬ 


line. A ray toward the 'sound is drawn from 
each mid-point making an angle 0 with the 
reference line, as determined by the relation: 

sin 6 = 

s 

where t = the distance between times of arrival 
at the two microphones, in seconds; 
s = the distance between microphones in 
sound-seconds. 



Figure 2. Sample galvanometer record for a six-microphone base, taken by U. S. Army in France. 


vals) and the arrivals of sound impulses are 
recorded. The time of arrival at each micro¬ 
phone, as measured from an arbitrary zero 
time, is read from the oscillogram, and the 
difference between arrival times is computed 
for each pair of adjacent microphones. 

Plotting. On the plotting board, the mid-point 
of each sub-base is plotted and the perpendicu¬ 
lar bisector is constructed for use as a reference 


One sound-second, the distance sound travels in 
1 second in air under accepted standard atmos¬ 
pheric conditions, equals 369.2 yd. The intersec¬ 
tion of the plotted rays (or average intersection 
of a polygon of error) is an approximate loca¬ 
tion of the source of sound. A more accurate 
location is obtained by applying curvature and 
weather corrections to the measured time 
intervals. 


CONFIDENTIAL 















































































































































































































































































































































































































































































































































































































































































































































































104 


GUN RANGING AND LOCATING SYSTEMS 


Number of Microphones. The number of mi¬ 
crophones installed depends on available time 
and terrain. A complete installation normally 
employs five or six microphones. A minimum 
of four microphones should always be installed 
to permit three-ray intersection at the target. 
Increasing the number of microphones increases 
the number of intersecting rays and improves 
the reliability of locations. There is also the 
possibility of one or more microphones becom¬ 
ing inoperative. 

Comparison of Bases. The most important 
advantage of a regular base (microphones uni¬ 
formly spaced) is that the recorded arrivals of 
sound at the microphones form an easily rec¬ 
ognized pattern on the oscillogram. Another 
advantage of a regular base is that standard 
plotting equipment with previously prepared 
time scales may be used. The curved regular 
base avoids the possible error of plotting a 
sound source to the front when it is actually 
to the rear, and facilitates oscillogram reading, 
because the recorded sound arrivals are grouped 
more closely together than for a comparable 
straight base. The curved base requires more 
computations in survey than does a straight 
base, and survey of a curved base is often more 
difficult to accomplish. This base was thus well 
adapted to the static fronts in World War I, 
but it was replaced in World War II by the 
regular straight base, which can be set up and 
used more quickly. Two such bases may be set 
up at right angles to each other, using one 
microphone in common. The irregular base is 
used when the terrain or time available for 
survey does not permit the installation of a 
regular base. It results in irregularity in the 
sequence of breaks, which, when there is con¬ 
siderable artillery activity, may render the os¬ 
cillograms unreadable. 

Length and Location of Base. The unit of 
measure for sub-bases is the sound-second. Con¬ 
venient sub-base lengths for rapid installations 
are 2 and 4 sound-seconds. However, other 
values, preferably multiples of a standard 
length, may be used. Satisfactory results may 
be obtained if the base length is not less than 
one-third of the range. The radius of a curved 
sound base is the distance in sound-seconds 
from the center of curvature to the arc through 


the mid-points. A radius is selected which will 
place the center of curvature near the center 
of the area to be observed, or which will fit the 
base onto the available terrain. A base should 
be as close as possible, consistent with the 
proper location of the outpost observer, to the 
area to be observed, and so oriented that the per¬ 
pendicular bisector passes through the approxi¬ 
mate center of the area. 

Outpost Positions. One outpost position should 
be at least 2 sound-seconds (approximately 750 
yd) closer to a sound source than any micro¬ 
phone. After the outpost observer detects the 
•sound, some delay, due to his reaction time, 
occurs before he depresses the starter key; 
there is a further delay in the operation of the 
relays before the sound set begins recording. 
For a short base, one outpost position may be 
sufficient. For a long base, two outposts, one 
toward either flank of the base, are necessary 
for complete coverage of a wide front, unless a 
single observer can be placed well forward. 

Sound Central. To reduce the amount of wire 
necessary, the sound central should be as near 
the center of the sound base as is consistent 
with security. Concealment for the personnel 
at the sound central and a covered route of 
approach should be provided. It may be desira¬ 
ble to install a sound base in which some or all 
of the wire circuits are replaced by a radio 
sound-data transmission system. 

Reading Oscillograms. Oscillograms are ob¬ 
tained by recording sound arrivals either elec¬ 
trically or photographically on a paper tape, 
at a rate of approximately 6 in. per second. 
On the tape, a number of horizontal lines are 
traced, one corresponding to each microphone 
installed. When no sound or wind strikes a 
microphone, the corresponding galvanometer 
trace is recorded as a straight line. Wind causes 
the line to waver from its normal position. 
When the sound of a gun reaches a microphone, 
an electric impulse is communicated to a gal¬ 
vanometer, producing a wavy line or break (see 
Figure 2). The point at which the trace first 
departs from its straight-line path, or zero line, 
is the initial break. With sound-ranging equip¬ 
ment now in use, the initial break is always 
downward for a sound beginning with a com- 
pressional wave, such as is produced by a gun 


CONFIDENTIAL 



LOCATION OF ENEMY EQUIPMENT BY SOUND 


105 


or bursting shell. In photographic recording, 
oscillograms are taken from the oscillograph, 
completely developed, and partially fixed. The 
fixing process may be completed by additional 
immersion for a few seconds in appropriate 
chemicals. Records may be read while wet, then 
dried for subsequent file. Both the galvanom¬ 
eter traces (horizontal lines) and the timing 
lines (vertical lines), as well as a serial number 
and the photograph of a 24-hour clock are 
recorded photographically on the oscillogram. 
The distance between adjacent vertical lines 
represents 0.010 sec. With this scale as a basis, 
the oscillogram reader determines the time of 
the initial break for each trace. Seconds, tenths, 
and hundredths are read directly, and thou¬ 
sandths are estimated by interpolating between 
the dots or lines. Recording equipment which 
is sufficiently sensitive to the higher frequencies 
may record a ballistic wave, which can readily 
be distinguished from a gun wave. Equipment 
less sensitive to higher frequencies may record 
the same sound, in which case the trace may be 
mistaken for that of a gun wave. Experience 
with a particular design of equipment will 
enable the observer to determine into which 
class the trace falls. The relative amplitudes 
of recorded gun and ballistic waves do not 
agree with their relative loudness as heard by 
an observer and may be misleading to an oscil¬ 
logram reader. 

Theory of Sound Plotting. The discharge of 
a gun or burst of a shell causes a pressure dis¬ 
turbance in the air which may be visualized 
as a constantly expanding hemisphere, with its 
center at the origin of the sound. If there is 
no wind, and the entire mass of air has a uni¬ 
form temperature of 50 F and uniform relative 
humidity of 50 per cent, the velocity of the ad¬ 
vancing wave front is 369.2 yd per second. 
These are the assumed “standard” conditions 
used in sound ranging. If it is assumed that the 
gun and all microphones are in the same plane, 
and the time readings for two adjacent micro¬ 
phones M x and M 2 (see Figure 1) are T x and T 2 , 
then the time difference ( T 2 — T i) multiplied 
by the velocity of sound in air V equals the 
difference in distances traveled by the sound 
from the source to each of the two microphones, 
V(T 2 — T i). Since by definition a hyperbola 


is the locus of all points the difference of whose 
distances from two fixed points is constant, the 
sound source lies on a hyperbola whose foci are 
the two adjacent microphones Mi and Mo and 
whose constant difference in distance is V- 
{To — T x ) . A hyperbola constructed from these 
values would pass through the sound source. 
A hyperbola constructed with M 2 and M 3 as 
foci and V(T s — To) as fixed difference would 
also pass through the sound source. The inter¬ 
section of the hyperbolas constructed for all 
pairs of adjacent microphones is the location 
of the sound source, but the physical construc¬ 
tion of so many curves is impractical in sound 
ranging. Associated with each hyperbola is its 
asymptote. At ranges normally encountered in 
sound ranging, the hyperbola and its asymptote 
are only a few yards apart, introducing only a 
very small or negligible error. The construction 
of the asymptote may be simply and rapidly per¬ 
formed for each time difference between ad¬ 
jacent microphones with the aid of a trans¬ 
parent fan, marked with a time scale for the 
particular base employed. In practice, after an 
approximate value for the range has been de¬ 
termined, a curvature correction for each sub¬ 
base, called the asymptote correction, is found 
by use of a chart. 

Weather Corrections. The methods described 
assume standard weather conditions, which sel¬ 
dom exist. To account for actual weather effects, 
corrections are applied to time intervals meas¬ 
ured under existing conditions to determine 
the intervals that would be recorded under 
“standard” conditions; rays are then plotted 
using the corrected intervals. In the standard 
method, correction formulas for wind and tem¬ 
perature are used. These formulas are based on 
physical principles, but involve the assumption 
of a horizontally stratified atmosphere and the 
so-called effective uniform wind and the effec¬ 
tive temperature. If the effect of humidity is 
also to be compensated for, the so-called virtual 
temperature is used. The effective wind speed 
is an empirically determined average of the 
wind velocities at various levels as measured 
by balloons observed with theodolites. The ef¬ 
fective wind direction is similarly determined, 
and the effective temperature is arbitrarily 
taken as 2 F below that at the ground surface. 


CONFIDENTIAL 



106 


GUN RANGING AND LOCATING SYSTEMS 


Polygon of Error. Even with accurate survey 
and complete curvature and weather correc¬ 
tions, a point plot (all rays of the sound plot in¬ 
tersecting at the same point) is seldom obtained, 
principally because exact weather corrections 
cannot be made. The polygon bounded by the 
intersecting rays is termed the polygon of error, 
which, when evaluated, determines the prob¬ 
able location of the sound source. Unless the 
polygon is very large or time is available for a 
more careful evaluation, this is accomplished 
by inspection. When the polygon is not evalu¬ 
ated by inspection, a unit weight is given to 
the intersection of any two rays; this simplifies 
procedure but does not take into account rela¬ 
tive strength of the intersection. If a grid has 
been placed on the sound plotting board, the 
coordinates of the probable location of the 
sound source may be read off directly. 

Accuracy of Location. The estimated accuracy 
of the location is the number of yards that the 
determined location of the target is estimated 
to vary from its true location. The accuracy of 
a sound-ranging location is somewhat better 
in deflection (measured from the perpendicular 
bisector of the base) than it is in range (meas¬ 
ured along the perpendicular to the base). Thus, 
a reported accuracy of 50 yd may represent a 
possible error of 50 yd in range, but only 20 
to 30 yd in deflection. 

522 Sound Waves Associated with Gunfire 

General. There are several distinct sounds 
associated with artillery fire, some or all of 
which may be produced when a single round is 
fired. Of these, only two, the gun wave and 
shell-burst wave, are of general use in sound 
ranging. A third, the ballistic wave, may have 
occasional use. 

Gun Wave. The gun wave, or muzzle wave, 
is the sound produced by the piece when it fires. 
Gases under high pressure suddenly released 
from the muzzle create a pressure wave. The 
shape of the wave front is initially quite com¬ 
plicated, but after the sound has traveled a 
short distance, the wave front becomes very 
nearly spherical, with its center of curvature 
a few yards ahead of the muzzle. Most of the 
energy of this sound is in the lower audible 


and subaudible frequencies. The wave form is 
smooth and, in general, the larger the gun, the 
longer is the period of oscillation. 

Ballistic Wave. A projectile whose velocity 
in flight is greater than the velocity of sound 
gives rise to a ballistic wave, or shell wave, anal¬ 
ogous to the bow wave of a ship. The presence 
of a ballistic wave is apparent to an observer 
near the line of flight of the shell because two 
distinct sounds are heard. The first, a sharp 
crack, is the ballistic wave. The second, a lower 
pitched boom, is the gun wave. 

Burst Wave. The burst wave, or detonation 
wave, is also an impulse wave, which originates 
in the bursting of a high-explosive shell. It is 
similar in some respects to the gun wave, but 
results from a violent detonation rather than 
from explosion of a slow-burning propellent 
charge. The energy of the detonation is distrib¬ 
uted over a wider range of frequencies. If 
sound-recording equipment is sensitive to 
higher frequencies, a more jagged record is 
obtained at short ranges. Because of greater 
attenuation of high-frequency sound, however, 
a record of a shell burst at a long range is 
almost identical in form to that of a gun wave. 

5 ' 2,3 General Requirements of the Project 

The study and improvement of standard 
sound-ranging methods and equipment involved 
practically every phase of the system. Methods 
of obtaining and handling the data were im¬ 
proved. In the development of new methods and 
equipment attention was given to the need for 
sound-ranging equipment better adapted to in¬ 
vasion tactics and a more fluid type of warfare. 
Finally, the need for reducing errors in sound 
ranging caused by weather and terrain was 
responsible for fundamental physical research 
on sound transmission above the ground. 

53 STANDARD METHODS AND EQUIP¬ 
MENT IN SOUND RANGING 

5,31 Obtaining the Data 

Methods 

Types of Bases. 2 In choosing the best type of 
microphone base, many factors must be con- 


CONFIDENTIAL 



STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


107 


sidered, such as ease of survey, number of 
microphones and amount of wire available, mo¬ 
bility, ease of camouflage, type of recorder used, 
ease of identifying sound sources on the record, 
ease of calculations, method of meteorological 
correction, and resulting accuracy. 

1. Curved Base. A curved base of circular 
form was used by the U. S. Army in World 
War I. This base requires more time to set up 
than a straight base, but once established it 
possesses certain advantages. For example, the 
symmetry is such that from a quick inspection 
of the arrival times at the various microphones 
one can estimate the general location of the 



Figure 3. Frequency-response characteristics of 
the T-21-B microphone and the GR-3-C recorder. 


enemy gun relative to the center of curvature 
of the base. 

2. Straight Base. The circular base was re¬ 
placed in World War II by a regular straight 
base. This type of base can be quickly surveyed 
in. Ease of plotting is another factor in its 
favor, but a quick estimate of the location of 
the enemy gun from inspection of the arrival 
times is less feasible than with the curved base. 

3. Triangular Base. The U. S. Signal Corps 
suggested and developed a sound-ranging sys¬ 
tem employing triangular sub-bases, 27 in each of 
which three microphones are placed at the ver¬ 
tices of an equilateral triangle. This arrange¬ 
ment gives better directional characteristics, 
and computed azimuths are independent of the 
velocity of sound and hence independent of the 
effective wind and temperature. A physical 
rather than an empirical method of correcting 


for the macromet was \yorked out for this base. 
Disadvantages are: the difficulty of identifying 
identical sound sources at the two isolated tri¬ 
angles or the necessity of coordinating the two 
triangular sub-bases to range on the same sound 
impulse; the more difficult surveying job; the 
increased amount of wire required; and the fact 
that if one microphone goes bad, the base be¬ 
comes inoperative. To counteract the first ob¬ 
jection, a combination of the triangular and 
straight bases was suggested by the Division 
for those cases where time and amount of wire 
available permit, e.g., on static fronts. 

It is found that different methods of applying 
a meteorological correction may be used for 
each type of base. These methods will be com¬ 
pared later. It was first assumed that, after 
corrections had been applied, the probable error 
in arrival time at each microphone due to the 
combined effect of all factors was the same for 
each type of base. The resulting error in loca¬ 
tion was then investigated. Under these cir¬ 
cumstances it was found that a single triangu¬ 
lar array is superior as a direction finder to a 
two-microphone array with the same separa¬ 
tion, and that a combination of two triangular 
bases gives a more accurate direction-and-range 
determination than a three-microphone straight 
base if the product of the length of a side and 
the separation between the centers of the tri¬ 
angles is greater than the square of the micro¬ 
phone separation along the straight base. In 
general, either an increased number or wider 
separation of microphones results in increased 
accuracy, and a two-dimensional array is more 
accurate than a one-dimensional setup. 

Equipment 

As explained in Section 5.2, the standard 
system of gun ranging utilizes a long, straight 
microphone base, one or two outpost observers, 
and a photographic recorder located at the 
sound central. Efforts of the Division were di¬ 
rected toward improving all three elements of 
the standard system. 

Modified T-21-B Microphone . 6 This study of 
microphones was undertaken to determine those 
characteristics of the standard sound-ranging 
microphones which would produce maximum 
ease of trace reading and maximum differenti- 


CONFIDENTIAL 


































108 


GUN RANGING AND LOCATING SYSTEMS 


ation of wave shape. The subjects investigated 
were: (1) the characteristics of standard sound¬ 
ranging microphones used in conjunction with 
standard recording devices; (2) the effect of 
microphone response characteristics on the re¬ 
corded wave; (3) the relationship between 
microphone response characteristics and ex¬ 
traneous sounds, and their effect on the wave; 
(4) modifications of standard microphones to 
obtain the greatest amount of information pos¬ 
sible from the record. 

Standard sound-ranging microphones. The 
microphone most widely used in sound-ranging 
operations in 1942 was the T-31-B. 28 This 
microphone was intended for use with the 
GR-3-C and similar sound-ranging sets. It is a 


ent in gun waves have been investigated by 
Kelley. 29 Representative values are shown in 
Table 1, which also includes data on the ballistic 
wave, and on the shell-burst wave associated 
with each major artillery piece. The frequency 
of maximum sound energy received varies with 
range, terrain, and meteorological conditions, 
but Table 1 shows clearly the advantages of a 
system having a response extending to a higher 
frequency than does the response of the stand¬ 
ard system with the T-21-B microphone (see 
Figure 3). 

Limitation due to noise. The extraneous noise 
encountered in field operations is principally 
caused by wind, but may also result from air¬ 
craft, vehicles, etc. Wind noise is of a random 


Table 1. Frequencies of sound waves produced by guns. 


Gun 

Muzzle Wave 

Ballistic Wave 

Shell-Burst Wave 



Frequency 




Frequency (c) of 

range (c) less 

Frequency (c) of 

Frequency (c) of 


maximum 

than 6 db 

maximum 

maximum 


energy 

down from 

energy 

energy 


component 

max. 

component 

component 

105-mm M-2 How. 

40 

15- 80 

90 

18 

155-mm M-l How. 

20 

5- 40 

32 

15 

8-in. M-l How. 

16 

6- 28 

46 

12 

240-mm M-l How. 

13 

6- 19 



4.5-in. M-l Gun 

20 

5- 40 

120 

20 

155-mm M-l Gun 

12 

4- 25 

94 

17 

.30-cal Mach, gun 

150 

50-350 



.50-cal Mach, gun 

120 

50-250 




combination condenser microphone and ampli¬ 
fier, mounted in a cylindrical container which 
forms a two-section, unterminated acoustic 
filter with damping built into each section. The 
overall amplitude-frequency characteristics of 
this microphone and the recorder are shown in 
Figure 3, Curve 2. (The recorder alone shows 
a response curve with a maximum at 20 c and a 
response down 6 db at 2 and 80 c.) A frequent 
source of equipment failure encountered with 
the T-21-B microphone under field conditions is 
in the condenser element, where the spacing 
between the diaphragm and stator plate is only 
0.004 in. The entrance of moisture or particles 
of dirt causes the element to become erratic. 
Repair of microphones so damaged is difficult. 

Effect of apparatus response on gun record. 
The range and magnitude of frequencies pres- 


nature and is not confined to any narrow band 
of frequencies, although the strongest com¬ 
ponents are at low frequencies. Noise from air¬ 
craft and vehicles in general increases with 
frequency in the region from 25 to 100 c and 
higher. These considerations impose an upper 
limit to the response a gun-ranging microphone 
should have. 

Modification of the standard microphone. As 
a first step in raising the upper cutoff fre¬ 
quency, new filter plugs were substituted, ex¬ 
tending the response to 35 c. To raise the upper 
cutoff frequency still higher, an acoustic coupler 
tube was developed (see Figure 4). This adapter 
seals the condenser element against the entrance 
of moisture and dirt by substituting a tube for 
the outer filter chamber and plug of the T-21-B; 
incorporated within the tube is a thin rubber 


CONFIDENTIAL 








STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


109 


diaphragm which effects the seal. The high 
flexibility and low mass of this diaphragm per¬ 
mit sound pressure to be freely transmitted to 
the condenser diaphragm. At the same time the 
rubber diaphragm provides mechanical loading 
of the acoustic space to raise the response to 
60 c. The inner filter plug of the T-21-B is re¬ 
placed by a suitable plug at the base of the tube. 
The desired frequency response is obtained by 
choosing suitable values for the diameter and 
thickness of the rubber diaphragm, the diam- 


standard T-21-B microphones, used with a 
GR-3-C recorder, were determined experimen¬ 
tally. In Figure 3 the results are shown for 
the modified (Curve 1) and the standard 
(Curve 2) microphones. The base of sound 
intensity is the same for the two curves. In 
further field tests a comparison was made of 
the wave shapes resulting when gun waves, 
ballistic waves, and shell-burst waves were 
recorded by the two microphones. Tests were 
also made to determine the effects of extrane- 



eter of the tube, and the hole dimensions of the 
lower filter plug. As a further means of pre¬ 
venting any appreciable amount of moisture 
from reaching the condenser element, a desic¬ 
cant chamber is provided. The air trapped 
between the rubber diaphragm and the con¬ 
denser can breathe through small holes to an 
outer chamber enclosed by a thin rubber bel¬ 
lows. Incorporated in the modification is a 
method of securing a frequency-response char¬ 
acteristic having a high-end cutoff at 100 c. 
This is intended for special use, such as mortar 
or machine-gun locating, and may be obtained 
by the removal of a screw from the center of 
the lower filter plug of the acoustic coupler tube. 

Advantages of the modified microphone. For 
purposes of comparison, the overall frequency- 
response characteristics of the modified and 


ous noises on each. The following conclusions 
were reached: 

1. The optimum overall amplitude-frequency 
response of the microphone and recording ap¬ 
paratus combined should have an upper cutoff 
of about 60 c for ranging on field artillery 
weapons, and of about 100 c for arms of smaller 
caliber such as machine guns. 

2. With a microphone having this 60-c re¬ 
sponse, a sharper break is obtained than with 
the standard 25-c response. 

3. The ambiguity between ballistic and muz¬ 
zle waves on the recorded trace is reduced by 
use of this higher cutoff response. 

4. Indications are that the extended frequency 
response is useful in securing information as 
to the type of gun being ranged (i.e., small, 
medium, or heavy). 


CONFIDENTIAL 
































































110 


GUN RANGING AND LOCATING SYSTEMS 


5. The increased amplitude of the recorded 
wave trace of all but the largest guns yields an 
improved signal-to-noise ratio, even though the 
response to extraneous noise is greater with the 
extended frequency range. 

6. Although the peak sensitivity of the modi¬ 
fied microphone is less than that of the stand¬ 
ard, the wider range of response maintains the 
effective sensitivity to actual gun sounds. 

7. The device used to incorporate a wider 
frequency response in the standard microphone 



Figure 5. Galvanometer unit for dry paper 
recorder. 


reduces certain inherent faults and increases 
its operating life. 

Dry Paper Recorder.™ The purpose of this 
work was to investigate the possibility of re¬ 
placing the standard photographic recorder of 
the GR-3-C with a small, lightweight, nonphoto¬ 
graphic recorder employing a strip of paper 
or similar material. Portability was a feature 
particularly desired by the U. S. Marine Corps. 
In this investigation the development of two 
recorder systems was initiated. The first was 


intended as a replacement for the GR-3-C, and 
the second was requested by the Marine Corps 
for use in its anticipated sound-ranging oper¬ 
ations. 

1. GR-3-C replacement. A dry paper recorder 
to replace the GR-3-C sound-ranging set was 
developed to the stage of a satisfactory labora¬ 
tory model; the work was carried far enough 
to prove the feasibility of the equipment. Fur¬ 
ther work toward its final design was in prog¬ 
ress but was stopped when it was learned that 
the Signal Corps Laboratory at Fort Monmouth, 
New Jersey, was developing similar equipment 
under contract with the Cambridge Instrument 
Company. 

A dry paper recorder using either wax-coated 
paper (furnished by Waxon-Carboff Company) 
or Teledeltos paper (distributed by Western 
Union Telegraph Company) was found practi¬ 
cal. However, data available at the time this 
work was abandoned indicated that the wax 
paper was superior to the Teledeltos. The wax 
paper is rated to withstand a temperature of 
125 F. The thinner grades were found to re¬ 
quire less pressure for marking, and less fric¬ 
tion was encountered when a heated stylus was 
used. 

It was found possible to mount six or eight 
galvanometers side by side, and to give a trace 
of sufficient amplitude by employing a recording 
paper 3 in. wide. The final form of galvanometer 
unit developed (shown in Figure 5) was capa¬ 
ble of being tuned to a natural frequency of 
70 c. When so tuned, its d-c sensitivity was 
0.033 in. per ma, and its response was essen¬ 
tially flat up to 30 c, with a 9 db peak at 
resonance. Damping was obtained both elec¬ 
trically and through the viscosity of the heated 
wax acting on the stylus. 

Satisfactory heater-type pointers for mark¬ 
ing on the wax paper were developed. 

In developing a marking mechanism for 
the time scale, a 100-c tuning fork was used 
to drive a synchronous motor. With such a 
mechanism 0.01-, 0.1-, and 1.0-second identify¬ 
ing marks could be made on the paper. The 
project was completed before the mechanism 
could be perfected. 

A paper drive mechanism, operated elec¬ 
trically at a rate of 5 in. per second, was de- 


CONFIDENTIAL 














STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


111 


veloped in laboratory form. The final model of 
the GR-3-C replacement is shown in Figure 6. 

2. Marine Corps recorder. A dry paper re¬ 
corder which was even lighter in weight and 
simpler in operation and maintenance was de¬ 
veloped for the Marine Corps to the stage of 
a laboratory model. Work was abandoned on 
notification from the Marine Corps that such 
sound-ranging facilities would be furnished 



Figure 6. Dry paper recorder: GR-3-C replace¬ 
ment. 


them by the U. S. Army Field Artillery Obser¬ 
vation Battalion. 

Teledeltos paper was used in this recorder. 
The marking was made through sparks from 
fixed pointers. Three pointers, spaced approxi¬ 
mately 0.06 in. apart, were used for each chan¬ 
nel. The marking voltage was normally con¬ 
nected to the central pointer, but the arrival 
of a positive sound pulse shifted the voltage 
to a side pointer. Thus the arrival times of 
sound pulses at the microphones of an array 
could be recorded, but not necessarily the char¬ 
acteristic wave traces of the pulses. 

The time scale was provided by a series of 
dots along both sides of the record. 

A paper drive with spring motor was devel¬ 
oped which ran for two minutes on one winding, 
or which could be operated continuously by 
winding during the operation. 

The Binaural Outpost. 13 The original object 


of this work was to develop a binaural listening 
device which could be used as a sound-ranging 
outpost in the standard gun-ranging method. 
Such a binaural outpost (Binop) might be used 
as a substitute for the outpost observer (as 
shown in Figure 7), or used in addition to him 
to enable the more highly trained personnel 
usually located at the sound central to check 
the work of the observer. 

A high-quality Binop covering a frequency 
range of approximately 50 to 9,000 c was de¬ 
veloped for the Field Artillery Observation Bat¬ 
talion outpost work. The Binop consists of the 
outpost unit and the sound central unit, as 
shown in Figure 8. The first is a two-channel 
sound pickup and amplifier mounted inside a 
case simulating the human head. Associated 
with it is a stake, on which the head is mounted, 
and a battery box for power. Binding posts are 
provided for terminating the wire transmission 
lines and a standard field telephone circuit. The 
sound central unit is a small, box-mounted con¬ 
trol panel with binding posts for terminating 
lines from the outpost unit, a field telephone, 
and a controlling device. Associated with this 

ENEMY | POSITION 



Figure 7. Use of binaural outpost in sound¬ 
ranging system. 


unit is a pair of high-quality head receivers. 
The Binop weighs approximately 39 lb, divided 
as follows: head, 23 lb; battery supply, 8 lb; 
stake, 5 lb; sound central unit, 3 lb. 

The channel from each microphone consists 


CONFIDENTIAL 







112 


GUN RANGING AND LOCATING SYSTEMS 


of an input transformer, a three-stage resist¬ 
ance-coupled amplifier with negative feedback 
around all three stages, and an output trans¬ 
former connecting the wire transmission line. 
The line is terminated by an impedance-match¬ 
ing transformer to a head receiver. One channel 
has means for adjusting the feedback voltage 
to the screen of the first vacuum tube to control 
the gain of that channel independently of its 
input gain control. The other channel has a 
fixed feedback. The gain of both channels is 
controlled simultaneously by a dual potentiom¬ 
eter unit in the grid circuit of the first tube. 


the resultant voltage across the head receivers 
will be zero. Therefore, by listening on the head 
receivers to a sound being picked up by the 
microphones and adjusting the gain of one 
amplifier by means of the feedback control until 
a minimum of sound is heard, the gain of the 
two amplifiers is equalized. Poling the wire 
transmission line to maintain the proper phase 
relationships between the signals in the two 
channels is accomplished in a similar manner. 

This outpost system was completed and tested 
by the Field Artillery Board. It performed the 
functions for which it was designed, but the 



The amplifier output transformer, the wire line, 
and the receiver impedance-matching trans¬ 
former are connected to provide a phantom 
circuit for telephone communication between 
the two ends of the line and for remote control 
of the amplifier filament supply relay from a 
switch at the receivers. 

Balancing the gains of the two channels is 
accomplished by means of a switch connecting 
the inputs to the two amplifiers. The output 
voltages of the two amplifiers are connected 
by a switch in series with a pair of head re¬ 
ceivers so that the voltages oppose each other. 
Under this condition, any sound picked up by 
the microphones will result in opposing voltages 
at the amplifier output; if the gains are equal, 


Board decided that these functions were not 
sufficiently valuable to warrant the transporta¬ 
tion of the necessary equipment. 


Handling the Data 

Methods 

Standard Plotting and Methods of Weight¬ 
ing. 2 In the sound plot of the standard method 
of sound ranging, rays (representing corrected 
asymptotes) are drawn from the mid-point of 
each sub-base, and these intersecting rays form 
the so-called polygon of errors or cat’s cradle. 
In standard practice the probable location of 


CONFIDENTIAL 


















STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


113 


the sound source is made either by inspection 
or by more careful evaluation, but in either 
case equal weight is given to each intersection. 
It was the purpose of this work to investigate 
what method of weighting the intersections 
would yield the best results. Two cases were 
considered: (1) all sources of error were 
lumped together in a time error (taken as 
1 millisecond) at each microphone; (2) the 
error for one microphone was taken as 10 milli¬ 
seconds, all other errors being neglected. 

In the case of a straight base with six micro¬ 
phones and the same error in arrival time for 
each, analysis of field records revealed that the 
error in location will be minimized by weighting 
the following equally: (1) the average of the 
four intersections of adjacent asymptotes; (2) 
the average of three intersections of asymptotes 
separated by one; (3) the average of the two 
intersections of asymptotes separated by two; 
(4) the intersection of the two outside asymp¬ 
totes. 

In actual practice the intersections of asymp¬ 
totes separated by one are preferred, because 
they are more reliable than the intersections 
of adjacent asymptotes, or of widely separated 
ones. Intersections of widely separated asymp¬ 
totes are less dependable because of nonuni¬ 
formity of “met” conditions and terrain. 

The effect of an error in the arrival time of 
a sound is greater the greater the distance be¬ 
tween source and microphone. 

It was found that for a five-microphone base 
with a 2-sound-second separation, if accurate 
location to within 150 yd (in range) at 10,000 
yd is desired for azimuths between 45 and 
90 degrees (the latter referring to the source 
on the perpendicular bisector of the base), no 
time error greater than 1 millisecond can be 
tolerated at any microphone. If there is an error 
of 10 milliseconds at any microphone, the maxi¬ 
mum range for which a 150-yd accuracy can be 
secured for the same azimuth range is reduced 
to about 5,000 yd. 

Analytical Method of Computing . 3 One of the 
functions of the physical research group of the 
Division was to study the problem of securing 
simpler and, if possible, more accurate methods 
of computing sound-ranging data. Considera¬ 
tion was given to ways of minimizing the effect 


of errors in the time intervals used to deter¬ 
mine location of sound sources, regardless of 
the causes of the errors. The purpose of the 
project was to eliminate the use of the standard 
plotting board and to avoid the inaccuracies 
present in such substitutes as wooden boards 
with strings. It was generally understood that 
the Field Artillery Observation Battalions con¬ 
sidered the standard plotting board very cum¬ 
bersome and difficult to move from one place to 
another. For this reason many battalions re¬ 
placed this board by a homemade substitute of 
wood, on which paper scales and strings were 
mounted. Tests conducted by members of the 
Division using such boards and sound-ranging 
records from Fort Bragg showed the inade¬ 
quacy of the simplified plotting boards. 

Two general methods were devised: (1) the 
analytical asymptote method and (2) the multi¬ 
polar coordinate method. 

In the analytical asymptote method, formulas 
were developed which give the rectangular co¬ 
ordinates of the points of intersection of the 
various pairs of asymptotes with respect to one 
of the microphones as a reference center. 
These formulas involve the microphone separa¬ 
tion, called 2a, and quantities d jk which are de¬ 
fined as half the products of the effective 
horizontal velocity of sound by the time inter¬ 
vals between arrival times at the microphones 
M k and M s . The resulting formulas are rather 
complicated, and tables were prepared to aid 
in evaluating the terms containing radicals. 
The formulas remain valid and workable as 
long as any three microphones of the straight 
base are in operation. No plotting is involved, 
so no errors can result from plotting inaccura¬ 
cies. Because this method uses the intersections 
of the asymptotes rather than of the hyperbolas 
themselves, a preliminary determination of the 
location of the sound source must be made to 
ascertain the appropriate asymptote corrections 
to be applied. Even though the use of suitable 
tables and charts decreases the amount of com¬ 
putation decidedly, a considerable amount 
of computation is still required—a serious dis¬ 
advantage. As a result of several trials, it was 
rather generally agreed that a method requir¬ 
ing less computation would be more desirable 
for field use. 


CONFIDENTIAL 



114 


GUN RANGING AND LOCATING SYSTEMS 


The multipolar coordinate method has the 
advantage of avoiding the asymptote correc¬ 
tion. Considered as a purely analytical method, 
it also requires excessive computation. It served, 
however, as the foundation for the development 
of a nomographic method which is felt to have 
considerable possibilities and which has evoked 
favorable comment in field trials. 

For both the analytical asymptote method and 
the multipolar coordinate method, the individual 
locations were averaged by a least squares re¬ 
duction as well as by taking an arithmetical 
average. The results did not show sufficient 
improvement to warrant the field use of the 
former method in preference to the latter. 

Nomographic Method of Computing .3,4,11,20 
In the multipolar coordinate method, the dis¬ 
tances of the sound source from each inner 
microphone of a regular straight base may be 
found from a formula which relates the dis¬ 
tances R lf R 2 , etc., to the microphone separa¬ 
tion 2 a and the difference in the times of ar¬ 
rival of the sound at the different microphones 
t jJk . Introducing the quantities \ jk = V H t jmkt 
where V H is the effective horizontal velocity of 
sound, one obtains by means of certain trigo¬ 
nometrical relations and the fact that distance 
is the product of velocity and time the formulas 

z > _ (X12 — 2 a) (X12 + 2 a) X12 ~t~ X23 

R2 ” X 12 - X 23 2 ( } 

Ri — R 2 — X 12 , R 3 = R 2 + X 23 ) etc. ( 2 ) 

The presence of only linear terms makes the 
R’s easy to compute, and there is no asymptote 
correction. The position of the source may then 
be obtained from the points of intersection of 
the circular arcs of radii R lf R 2 , etc., drawn 
from each microphone M lf M 2 , etc., as centers. 
(It is from this that the method takes its name.) 
To avoid constructing the circular arcs, an 
analytical alternative was worked out whereby 
the coordinates of the intersections of each R 
with every other R may be computed from 
formulas. These formulas are, however, too 
complicated for practical field use. 

Nomographic version of multipolar coordi¬ 
nate method. The next step in the improvement 
of the multipolar coordinate method was the 
development of equations in a form appropriate 
for constructing a nomogram to be used in 


determining locations from data obtained by 
standard sound-ranging methods. Consider 
three microphones, M lf M 2i and M 3 , equally 
spaced along a straight line. Let R be the de¬ 
sired range of the sound source relative to the 
central microphone, and 0 the azimuth or angle 
which the range vector makes with the normal 
to the base line. Then, from the same principles 
as were referred to in the preceding paragraph, 
two basic equations may be derived relating R 
and 0 to the differences in arrival times of the 
sound at the microphones. These basic equa¬ 
tions are 

A 2 + ^ - bA = 0 (3) 

V H \ VH / 

and 

4 a sin 0 BV H /.x 

~AVh~ ~ 2 R~ { } 

where 2a is the microphone separation, V H the 
horizontal effective velocity of sound, and A 
and B are defined as follows: 

A = tl-2 + ^2-3j B = —tl-2 + I 2 S (5) 

A and B are determined from the time differ¬ 
ences; then, for a given A and B, R may be 
found from equation (3) ; and finally, with R 
known, 0 may be found from equation (4). It 
was believed that these steps of computing R 
and 0 from equations (3) and (4) could be 
done most easily by means of a nomogram. Two 
forms of the resulting nomogram are shown 
schematically in Figures 9 and 10. Along the 
left edge of each is an A scale for which dis¬ 
tances from the base are proportional to A 2 , 
and along the right edge is a range scale which 
is linear in R. The sloping B curve in each is 
constructed in accordance with a rather elabo¬ 
rate formula. Two identical linear A scales are 
shown at the top and bottom in Figure 9, and 
one such scale near the bottom of Figure 10. At 
the bottom of the latter is an azimuth scale, 
whose distances are proportional to sin 0, with 
additional B scales on each side. Throughout 
the major portion of Figure 9 are constant- 
azimuth curves. These are obtained by elim¬ 
inating R between equations (3) and (4) and 
then, for a chosen 0, finding values of B for dif¬ 
ferent values of A, each pair of A and B values 
giving one point on the given constant-azimuth 


CONFIDENTIAL 







STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


115 


curve. To use the nomogram of Figure 9, the 
predetermined value of A on the left edge is 
connected with the predetermined value of B 
on the B curve. The extension of this line EE 
to the range scale gives the range. With the 
same value of A, corresponding points on the 



Figure 9. Nomogram schematic—nomogram with 
constant-azimuth curves. 


upper and lower A scales are connected. By 
reading the value in the field of constant-angle 
curves of the intersection of this line EE and 
the one previously drawn, the azimuth is found. 
An alternative method of obtaining the azimuth 
is to make use of the 6 scale at the bottom of 
the nomogram of Figure 10 in conjunction with 
the associated A and B scales. This method is 
expected to be used in only the relatively few 
cases—chiefly when the sound source is well on 
the flank at short range—for which the first 
method is inadequate. In this alternative 
method, the range is determined as before; 
then this value of R is connected to the value of 
A on the bottom A scale. The intersection of 
this second line KK, extended, with a horizontal 
line through the correct value of B on the ver¬ 
tical B scale gives a point whose projection on 
the azimuth scale is the corresponding value 
of 6 . 

Case of one microphone inoperative. Nomo¬ 
grams were also computed for the case where 
the separation between the second and third 


of a group of three microphones is twice that 
between the first and second, and for the case 
where the separation between the first and 
second microphone is twice that between the 
second and third. Such situations arise when 
one of the inner microphones of a four- to six- 
microphone base becomes inoperative. In this 
case two nomograms are necessary, in addition 
to the standard one for the completely operative 
base, to take care of all possible cases which 
may arise. 

Results of plotting by the nomographic 
method. For a five- or six-microphone base, 
R and 6 values may be found for each group 
of three microphones. If azimuth lines and 
range points are located on the plotting grid, 
the location of the sound source may be taken 
as the point halfway between the average of 
the range points and the average of the inter¬ 
sections of the azimuth lines. However, in view 
of the results of some tests conducted by the 
Field Artillery Board, Fort Bragg, North 
Carolina, it appears that more weight should 
be given to the intersections of the azimuth 



Figure 10. Nomogram schematic—nomogram 
with lower B scale. 


lines than to the range points, the former being 
much less affected by slight errors in reading 
the film, etc., than the latter. A comparison of 
the accuracy of location determined by using 
the standard method, the analytical asymptote 


CONFIDENTIAL 































































116 


GUN RANGING AND LOCATING SYSTEMS 


method, and the nomographic method showed 
that, so far as field operations are concerned, 
the nomographic method is the best. 

Ballistic-Burst Method. 12 The purpose of this 
work was the development of a new method 
of sound ranging which would utilize the 
arrival times of the ballistic wave and the shell- 
burst wave at the microphones of a sound base 
of the type used in the standard sound-ranging 
method. The development was started on 
August 16, 1943, at the suggestion of the Field 
Artillery Board. It was pointed out at that time 
that reports from Observation Battalions oper¬ 
ating in Tunisia indicated that sound records 
were frequently obtained on which the gun 
wave was inaudible and only the ballistic wave 
and shell-burst wave arrivals were recorded. 
In view of this fact, the Field Artillery Board 
was of the opinion that an investigation should 
be initiated to determine the possibility of 
obtaining information regarding gun locating 
using only the ballistic wave and shell-burst 
wave data. It was felt that even a determination 
of the azimuth of the gun position would ma¬ 
terially reduce the area which would have to 
be searched. A rough determination of the 
range would still further reduce the aerial 
search problem, even though the approximate 
sound location taken alone might not be suffi¬ 
ciently accurate for direction of counter-battery 
fire. 

Conditions for the use of the method. When 
the “met” measurements indicate that a nega¬ 
tive gradient of sound velocity with height 
exists in the direction from the gun to the base 
line, the conditions are rather unfavorable for 
sound ranging by the standard method on 
sources at large distances from the base. Even 
though the conditions are extremely unfavor¬ 
able for the transmission at long range of a 
gun wave originating at ground level, it is still 
very probable that the ballistic wave and shell- 
burst wave will be audible at the base, since 
the ballistic wave is generated by the shell at 
a point considerably closer to the base line and 
some distance above the surface of the ground, 
whereas the shell-burst wave will ordinarily 
originate from a point much closer to the base 
than the gun position. Frequently, the shell 
burst occurs behind the microphone base, so 


that the sound-velocity gradient is favorable 
for transmission back to the base. However, 
it should be emphasized that this method is 
not to be considered a replacement for the 
standard method of sound ranging but rather 
an extension of it, to be used in those cases 
in which the standard method does not give 
satisfactory results. 

Line-of-flight determination. The determina¬ 
tion of the line of flight of a shell is based on 
the fact that the trace of the ballistic wave 
on the ground is symmetrical with respect to 
the line of flight. The steps are as follows: 

1. It is first necessary to construct the trace 
of the ballistic wave on the ground. This trace 
is constructed by selecting the earliest arrival 
time as a zero reference and measuring the 
time intervals between this reference and the 
arrival times at all the other microphones. 
These time intervals are corrected to standard 
temperature of 50 F and converted into dis¬ 
tances. These values represent the distances 
from the various microphones to the ballistic 
wave front at the time the wave front was 
incident on the microphone at first arrival. 5 

2. If circles are drawn around the various 
microphones with radii equal to the correspond¬ 
ing distances obtained above from the time 
intervals, then the instantaneous position of 
the trace of the ballistic wave on the ground 
is a curve tangent to all these circles and pass¬ 
ing through the zero reference microphone (see 
Figure 11). 

3. Next, the shell-burst position is found by 
means of the shell-burst wave arrival times, 
using the standard method of sound ranging. 

4. After the shell-burst position is located, 
the line of flight is found by striking an arc 
with center at the shell-burst location and 
radius such that the arc intersects the ballistic 
ground-trace curve in two points, separated by 
a distance equivalent to about 2,000 yd at the 
scale of the map. 

5. With these two points as centers, two 
intersecting arcs of equal radii are drawn. The 
intersection of these two arcs determines a 

b A certain degree of approximation is assumed here 
since the ballistic wave does not approach the micro¬ 
phones horizontally and wind effects are neglected. 
However, investigation showed that the above method 
is sufficiently accurate for sound ranging. 


CONFIDENTIAL 




STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


117 


second point on the line of symmetry. Thus the 
line connecting the shell-burst position and this 
second point is the line of flight of the shell 
(see Figure 11). 

6 . It should be noted that this method can 
be used only if the line of flight crosses the base 
line; however, it is possible to treat the case 
in which the line of flight passes within one 
sub-base length of the end of the base. 

Range determination. After the method of 
determining the line of flight had been com¬ 



pleted, an investigation was started to deter¬ 
mine the feasibility of obtaining a value of the 
range. 

The time interval between the arrival of the 
ballistic wave and the shell-burst wave at the 
point of intersection of the line of flight and 
microphone base line can be calculated. It is 
possible, though not very likely, that two guns 
with different muzzle velocities and total ranges 
might give the same time interval; another pair 
of different guns might give identical ground 
traces of the ballistic wave through a given 
point. Thus, neither the time interval alone 
nor the shape of the ground trace alone com¬ 
pletely specifies the gun type, muzzle velocity, 
and range. However, it is very improbable that 
two different guns which give similar ground 
traces would also have equal time intervals 
between the arrivals of the ballistic and shell- 
burst waves, or vice versa. Thus it would seem 
that a consideration of both the shape of the 


ground trace and the time interval would be 
sufficient to determine uniquely the type of gun, 
muzzle velocity, and total range. This was the 
case for the German guns. 

To summarize, the following data available 
to the sound-ranging unit are in excess of those 
required for determining the line of flight: 
(1) distance d from shell burst to the ground 
trace of the ballistic wave; (2) time interval 
between arrival of the ballistic wave and shell- 
burst wave (A t 8B ) ; (3) curvature of the bal¬ 
listic wave ground trace. Corresponding to 
these three observable quantities are the follow¬ 
ing unknowns: (1) type of gun, (2) muzzle 
velocity (charge), and (3) total range. Though 
it should be possible to solve for the three un¬ 
knowns in terms of the observable quantities, it 
was found that the mathematical relationships 
between the observables and the unknowns 
were too complicated to be put in a simple form 
for field use. It was therefore necessary to 
resort to a graphical solution of the problem. 

This solution took the form of a series of 
templates giving the shape of the ballistic wave 
ground trace, for various ranges, for a par¬ 
ticular type of gun, charge, and distance d 
from shell burst to ballistic wave ground trace. 
(For a sample template see Figure 12.) For 
each gun and charge, templates were con¬ 
structed for particular values of d to cover the 
region within which shell bursts might nor¬ 
mally be expected. 

In addition to these templates, a set of tables 
were made up, giving the time interval between 
the arrival times of the ballistic and shell-burst 
waves as observed at the intersection of the 
base line and the line of flight. These time 
intervals were tabulated for various guns, 
charges, and ranges for particular values of the 
distance d. 12 

If the type of gun is known from other intelli¬ 
gence reports, the solution for the range to the 
gun is found by using the template appropriate 
to that type of gun and the range curve on that 
template which fits the plotted ballistic wave 
ground trace. 

If the type of gun and muzzle velocity are not 
known, it is necessary to repeat the procedure, 
using templates for each of the possible com¬ 
binations of gun type and muzzle velocity for 


CONFIDENTIAL 






118 


GUN RANGING AND LOCATING SYSTEMS 


which data are available. The resulting list of 
range values corresponding to these cases 
could be quite formidable, but the number can 
usually be considerably reduced by experience 
and the aid of intelligence reports. 



Figure 12. Sample template for giving shape of 

the ballistic wave ground trace. 

When the list of possibilities has been nar¬ 
rowed down as much as possible, the ballistic- 
burst time interval tables are used to determine 
the theoretical time intervals for the various 
combinations of gun, charge, and range which 
have not been eliminated. A comparison of 
these values with the experimental time inter¬ 
val should reveal a single combination of gun, 
charge, and range. 

The use of the ballistic-burst time interval 
involves more approximation than exists in the 
use of the templates. This accounts for the use 
of the templates for the range determination, 
whereas the time interval is used as a qualita¬ 
tive check to determine the type of gun and 
charge and not for a quantitative determina¬ 
tion of the range. 

A method of applying ballistic and meteoro¬ 


logical corrections to this method of sound 
location was developed which corrects for the 
effects of meteorology and drift on the trajec¬ 
tory, and for the effects of meteorology on the 
sound transmission of the shell-burst wave. 
However, it is not possible to make any correc¬ 
tion for variations in muzzle velocity due to 
wear of the gun, since this data is not available 
under battle conditions. This results in an 
uncorrectable error in range—the limit of the 
accuracy of the method. 

Extensive field tests were carried out at Fort 
Bragg, North Carolina, which proved the 
greater efficiency of this method over the 
standard method under favorable “met” condi¬ 
tions. The tests further showed that the method 
served the purpose for which it was developed. 
The error in line or azimuth was estimated to 
be about ±10 mils (6,400 mils = 360 degrees), 
as seen from the shell-burst position, for an 
azimuth of not more than 30 degrees. Thus one 
dimension of the rectangle of error is 1 per 
cent of the observed range. The error in range 
is mainly determined by the unknown wear 
of the enemy gun. In the absence of any infor¬ 
mation from other sources on the wear of the 
gun, this error was estimated at about ±500 
yd for ranges of 20,000 yd or greater. The 
residual experimental error of the method was 
about ±100 yd. Thus this method is compar¬ 
able in accuracy with the standard method for 
ranges over 20,000 yd; for short ranges, how¬ 
ever, it is less accurate, and should be used 
only as a rough check when results of the 
standard method seem doubtful. 

Study of “Met” Corrections. 2 ' 10 The errors 
in sound ranging fall into two classes: instru¬ 
mental errors and propagation errors. The 
propagation errors include the effects of 
meteorology and terrain on sound waves. Thus 
in the standard method of gun ranging a 
meteorological correction must be applied to 
each measured difference in arrival times. The 
techniques of and equipment for acoustic gun 
ranging are sufficiently advanced so that the 
instrumental errors are smaller than or, at 
most, of the same order of magnitude as the 
propagation errors. Therefore, a study of the 
best method of applying a meteorological cor¬ 
rection to data obtained by the standard method 


CONFIDENTIAL 







STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


119 


was considered important and was undertaken 
by the Division. 

Various methods were compared; the follow¬ 
ing were regarded as important enough for 
special study: (1) the standard correction now 
in use by the U. S. Field Artillery for a straight 
base; (2) the Goodwin method developed in 
Great Britain for use with a straight base; 
(3) the U. S. Army Signal Corps triangular 
array method; 30 (4) the V H method developed 
by the Second Field Artillery Observation Bat¬ 
talion. 

The standard meteorological correction 
method is described in Field Artillery Field 
Manual FM 6-120 and has been outlined briefly 
in Section 5.2.1 of this report. It is used with 
a straight base and the GR-3-C recording sys¬ 
tem and (in a modified form) with the Dodar 
system (see Section 5.4). The method was 
critically studied by members of the Division; 
in general, application of meteorological cor¬ 
rections by this method was found to improve 
the results statistically, but the effectiveness 
varied considerably in individual cases, and in 
some cases was very inadequate. Two serious 
objections are pointed out: (1) in determining 
the effective wind speed and direction, the 
wind velocities at the various heights should 
be weighted as vector quantities; (2) the de¬ 
termination of the effective temperature is too 
arbitrary. A reconsideration of the weighting 
factors based, if possible, on theoretical con¬ 
siderations, is recommended. 

The Goodwin method 31 is more elaborate, 
being essentially a numerical calculation in the 
calculus of variations. It also requires the 
preparation and use of a set of tables. How¬ 
ever, its formulas are based entirely on theo¬ 
retical considerations, assuming again a steady 
and horizontally stratified atmosphere. Two ob¬ 
jections to this method are its complexity and 
the fact that only the wind component parallel 
to the plane of propagation is considered, cross- 
winds being neglected. 

The Signal Corps method is applicable to the 
triangular base. As in the Goodwin method, 
rather elaborate calculations are called for; this 
method is also based on theoretical considera¬ 
tions and the assumption of a steady and hori¬ 
zontally stratified atmosphere. A fundamental 


element in this method of correction is the 
so-called criterion curve which is a plot of 
V(z) + w(z) versus z, where V (z) and w(z) 
are, respectively, the scalar velocity of sound 
and the wind component in the plane of the 
sound ray at the height z above the ground. 
This curve is obtained from a vertical sounding 
of the atmosphere. An immediate advantage of 
obtaining the criterion curve is that one may 
determine whether or not there are multiple 
paths. The sound path may be plotted in each 
case. The atmosphere is divided into horizontal 
layers, and the horizontal distance traveled 
and drift determined for each layer. Thus 
allowance is made for cross-winds. The Signal 
Corps method is also independent of the pos¬ 
sible existence of multiple reflections at the 
ground in the transmission from source to 
microphone base, whereas the other methods 
are not. 

The V H method is essentially an attempt to 
apply some of the features of the Signal Corps 
method (notably the criterion curve) to the 
standard straight or curved base. This necessi¬ 
tates placing two auxiliary microphones behind 
the standard base to give the equivalent of a 
straight base and two triangular bases. The 
auxiliary microphones make it possible to com¬ 
pute the effective horizontal velocity of sound 
at each end of the standard base. Intermediate 
values are computed by interpolation. The ar¬ 
rival times at each microphone can then be 
corrected to what they would be for standard 
conditions (no wind and a temperature of 
50 F). Furthermore corrections for drift due 
to cross-winds may be computed, if desired, as 
in the Signal Corps method. 

A comparative test of the four methods out¬ 
lined above was made using a rather small 
amount of data obtained during actual artillery 
fire for which the true location of the source 
was known in each case. The more elaborate 
methods proved to be little, if at all, superior 
to the standard Field Artillery method. These 
results are in agreement with those of a some¬ 
what similar comparison made by the British. 32 
It is possible that the Signal Corps or the V H 
method may be superior for long-range loca¬ 
tions in which overhead paths may be im¬ 
portant. From the theoretical point of view, 


CONFIDENTIAL 



120 


GUN RANGING AND LOCATING SYSTEMS 


these two methods would seem to be preferable; 
but on the practical side, one must consider 
such factors as ease of computation, availability 
of sufficient and accurate meteorological data, 
the actual meteorological structure, and the 
type of base which is easiest to set up and 
maintain. 

Equipment 

Trace-Reading Templates. One of the sources 
of error involved in the transfer of data from 



the sound-ranging record to the final plot lies 
in the determination of the exact time of the 
initial break on the record. The nature of this 
break varies over wide ranges with respect to 
the sharpness of the initial inflection. When 
breaks are sharp, an experienced film reader 
can usually determine them to within 0.002 
second. Under usual sound-ranging conditions, 
this degree of accuracy results in only minor 
errors in the plot. Very often, however, the 


time breaks are quite rounded, and even ex¬ 
perienced film readers may vary as much as 
0.010 second in their estimates. Errors of this 
magnitude can result in range errors up to 
150 yd and more for ranges of 10,000 yd and 
over. Inexperienced film readers, who may 
necessarily be utilized under battle conditions, 
can increase this error considerably. In an 
attempt to reduce the error resulting from 
visual film reading, several types of reading 
scales were devised and tried in the field. 

The reading scale devised to assist in the 
accurate determination of the initial breaks 
consists of a series of varying-sized quadrants 
of circles with tangents extended from one end, 
as shown in Figure 13. In use the proper “hook” 
is determined by matching the curvature of the 
hook to the particular gun break. The scale is 
then rotated so that the tail lies tangent to the 
steepest part of the wave. The time of first 
arrival is then read at the dot or center of the 
quadrant. The reading scale is adaptable to any 
amplitude of break since variations in ampli¬ 
tude merely rotate the hook about its center or 
dot. 

In addition to the series of hooks, a straight 
line having crosslines is included on the read¬ 
ing scale. This may be used to determine the 
point of first crossover of the gun wave con¬ 
veniently and accurately. 

Field tests of the reading scale proved that 
a substantial improvement in accuracy and 
especially uniformity results from its use. On 
a series of gun waves with varying degrees of 
curvature, three film readers, estimating with¬ 
out the aid of the reading scale, varied in their 
estimates of the sharpest breaks by 0.002 
second, and of the most rounded breaks by 
0.008 second. The same three readers, using 
the reading scale, located the same breaks with 
a variation of only 0.001 second for the sharp 
breaks, and 0.002 second for the most rounded. 
This indicates much greater consistency in 
reading by different observers. 

Over 800 copies of this template, printed on 
Lucite, were manufactured and furnished the 
Field Artillery Board. Some of these copies 
were flown to the fighting fronts. 

Nomogram and Accessories—The Plotting 
Grid. 11 ' 15 ’ 20 The theory of the nomogram, with 


CONFIDENTIAL 





STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


121 


schematic diagrams illustrating its method of 
use, has been explained in Section 5.3.2. Nomo¬ 
grams were computed and master drawings 
were made for the following: 2-sound-second 
straight base, 4-sound-second straight base, 
and 4-sound-second straight base with inop¬ 
erative inner microphone (two nomograms 
required to take care of all possible situations). 
These nomograms may be used for any uniform 


in mind: (1) the equipment must be light and 
portable; (2) the equipment must be rugged 
and corrosion-proof; (3) sliders must move 
easily and be self-locking when released; (4) 
scale settings must be accurate, with special 
means to avoid parallax. 

All these requirements were met in the final 
models supplied to the Field Artillery Board 
for field tests; at the close of the contract, how- 



Figure 14. Final nomogram mechanism with mounted nomogram chart. 


microphone separation on a straight base by 
use of suitable multiplying factors. An investi¬ 
gation was next made of the best method of 
making prints of the above nomograms and 
of the best type of base and reading mechanism 
on which to mount such prints. The develop¬ 
ment of an improved method of plotting 
necessitated the design of auxiliary plotting 
equipment. Finally a suitable case for carrying 
all the above equipment was designed and built. 

In the development work just outlined the 
following military requirements had to be kept 


ever, design for quantity manufacturing had 
not been started. 

Nomographic equipment included the follow¬ 
ing: 

1. Four nomogram charts. These charts, 
about 20 by 16 inches, were printed on Lucite by 
the Sweeney Lithograph Company, Belleville, 
New Jersey, using a special ink developed by 
this company for the purpose. The charts are 
coated with “Sweeney Hardcoat” for protec¬ 
tion. On the back of each chart is printed a grid 
on a scale of 1 to 25,000 (in yards). This coating 


CONFIDENTIAL 












































122 


GUN RANGING AND LOCATING SYSTEMS 


could be made suitable for marking and erasing, 
and it proved to wear well. 

2. A nomogram base and reading mechanism 
(shown in Figure 14 with one of the charts 
mounted on it). The base is made of stainless 
steel. The sliders on the sides are connected 
by two extendible nylon filaments, one above 
the other so as to eliminate parallax. The top 
and bottom sliders move together, through an 
arrangement of cables and pulleys, so as to keep 
the filament stretched between them always at 
right angles to the top and bottom A scales. 

3. A portable microphone base to be used 
with plastic plotting grids. This base is made 
of Lucite and can be attached to the plotting 
board by means of vacuum cups. 

4. A device for drawing normals to the base 
line, also of Lucite. 

5. A protractor or fan, similar to the stand¬ 
ard plotting fan, but made of Lucite. 

6. A carrying case. This can be used as a 
plotting table. The case measures 29V2 by 
23% by 3% in. and, together with the above 
equipment, weighs 33 lb. Future plans con¬ 
templated an all-welded case with gaskets and 
weather-protecting paint. 

Advantages of the nomogram: 

1. The nomographic equipment is much 
lighter than the standard M-l sound-ranging 
board. 

2. The preliminary plot for curvature cor¬ 
rection required in the standard method is 
eliminated. 

3. The range determinations, being much 
more sensitive to fluctuations in atmospheric 
sound transmission than are the azimuth de¬ 
terminations, afford a means of judging the 
type of weather, terrain, etc. 

4. The accurate methods of plotting devel¬ 
oped in connection with the nomogram proved 
sufficiently superior to standard methods so 
that a request was made to extend the develop¬ 
ment of plotting methods to include all Field 
Artillery plotting (see the next section). 

5. Extensive field tests showed that the 
nomogram will yield results of a high degree 
of accuracy, provided gross human errors are 
eliminated. (In the tests at Fort Bragg, such 
errors played a considerable role, causing the 


nomogram to make a poorer showing than did 
the standard methods.) 

Artillery Plotting Grids. 1 * In standard plot¬ 
ting practice, points are marked on translucent 
paper which has been superimposed on a cross- 
section paper map. All forms of paper used for 
these grids and for plotting the location of 
enemy targets are subject to changes in size 
and shape as a result of changes in humidity 
and temperature. This inherent warping results 
in errors of location of as much as ±50 yd 
at a range of 20,000 yd. The development of 
Lucite grids for use with the nomogram and 
the necessity for extreme accuracy led to the 
development of such grids for all artillery 
plotting. These grids consist of rectangular 
sheets of Lucite ruled on one face with squares 
whose sides represent 1,000 yd, usually on a 
scale of 1 to 25,000. The other side is similarly 
ruled in 100-meter squares. The grids were 



Figure 15. Carrying case with grids and acces¬ 
sory plotting equipment. 

housed in a suitable carrying case (see Figure 
15). 

The results of laboratory tests and field tests 
at Fort Bragg showed that the Lucite grids 
ensure greater accuracy than any plotting 
means yet developed. These grids, together with 
Lucite plotting fans and Lucite locators 
mounted on vacuum cups, have been found 


CONFIDENTIAL 
















STANDARD METHODS AND EQUIPMENT IN SOUND RANGING 


123 


to reduce the errors in location due to warping, 
etc., to an average of ±10 yd, thus reducing 
the cost of neutralizing an enemy target by 
a factor of from 1 to 25. 

The Lucite grids and accessory equipment 
provide an important time saver, with the de¬ 
sired flexibility necessary for the quick firing 
encountered in mobile warfare. 

They may be used for plotting target loca¬ 
tions on their surfaces. Location marks can 
be readily removed by erasing, without injury 
to the surface. 

Field tests indicated that contact with grit, 
dirt, leaves, and even rain does not impair the 
usefulness of the grids. 


burst to the ground trace of the ballistic wave, 
the problem was to determine the ballistic wave 
ground trace for' each of a series of ranges of 
the gun. Since no sufficiently accurate equation 
for the coordinates of an actual trajectory is 
in existence, it was necessary to resort to a 
graphical method of solution. 

A series of trajectories was constructed, 
spaced at 1,000-yd intervals in total range 
throughout the range values to be considered. 
Each of these trajectories was specified by the 
coordinates of points at 1-second intervals 
in shell travel time up to the point at which 
the shell velocity dropped below the velocity of 
sound. A sample chart of this type is shown in 



Figure 16. Sample trajectory chart. 


Ballistic-Burst Templates and Tables. 12 In 
the description of the ballistic-burst method 
the need for templates and tables prepared for 
each gun in question was explained. Construc¬ 
tion of these templates and tables was possible 
only by a graphical method involving much 
laborious work. For each gun the following 
data had to be compiled: (1) the coordinates 
of the time points for the trajectories of the 
gun (provided by the Aberdeen Proving 
Ground), and (2) the firing tables for the gun 
(furnished by the Field Artillery Board). 

Templates and tables were prepared for the 
German 170-mm K18, the German 210-mm Mrs 
18, and the American 155-mm M-l. In con¬ 
structing a template for a given gun, charge, 
and fixed value of the distance d from shell 


Figure 16. In plotting the trajectories it was 
convenient to make the shell-burst position 
common for all trajectories, since this is the 
common reference point for any given template. 
The adjacent points on a given trajectory were 
connected by straight-line segments for con¬ 
venience in following a given trajectory, and 
the range in 1,000-yd units was marked at 
several points. These connecting lines were for 
guidance only. In a similar manner the points 
of equal time on adjacent trajectories were 
also connected by straight-line segments, and 
the time value in seconds marked at several 
points. This double family of curves formed a 
convenient coordinate system for locating a 
particular time point on any given trajectory. 

The construction of the templates required 


CONFIDENTIAL 







































124 


GUN RANGING AND LOCATING SYSTEMS 


the projections of all the trajectory points on 
the horizontal or x axis. When a large number 
of trajectories were plotted on the same chart 
(Figure 16), the array of vertical lines became 
very difficult to follow; therefore, the following 
method was adopted. A series of equally spaced 
lines was drawn parallel to the horizontal axis 
and just below it. Each one of these lines was 
numbered to correspond to one of the trajec¬ 
tories plotted on the chart, every fifth line being 
made heavier. The projections of the points of 
a given trajectory were then marked on the 
corresponding horizontal line, and a short 
vertical line drawn back from these projection 
points to the x axis to determine the actual 
projection point on this axis (bottom of Figure 
16). The advantage of this system is that the 
projection of a given time point for any par¬ 
ticular trajectory can be selected by eye from 
the horizontal line corresponding to this tra¬ 
jectory. When this point has been found, it is 
necessary merely to follow the short vertical 
line from this point to the x axis, a distance 
never exceeding 2 in. Experience showed that 
this method of projection materially increases 
the speed of the construction process. 

On the sheet which was to be used for the 
master template (see Figure 12) a line was 
drawn parallel to the upper edge and about 
half an inch from this edge. This line repre¬ 
sented the line of flight with reference to which 
the template curves were to be constructed. 
About half an inch from the right edge of the 
template sheet a short vertical line was drawn 
across the line of flight. The intersection 0 
of this line and the line of flight was taken as 
the reference point or vertex of all the curves 
on the template. The template sheet was placed 
on the trajectory chart in such a manner that 
the line of flight coincided with the x axis of 
the trajectory chart; the short vertical line 
was set at the particular distance d from the 
shell burst for which this template was to be 
constructed. (This sheet was located so that 
the main construction area extended below the 
x axis and would not cover any of the trajectory 
points.) 

The ballistic wave ground traces passing 
through the reference point 0 on the suitably 
located template sheet were constructed by 


locating first the apparent ballistic origin for 
this point on a given trajectory. This apparent 
ballistic origin was that point on the trajectory 
at which the component of the shell velocity in 
the direction of the reference point was exactly 
equal to the velocity of sound. 

By means of a compass with center at the 
apparent source for the reference point 0, an 
arc was struck with radius such that it passed 
through the point 0. The center was then moved 
back one second on the trajectory, the radius 
increased by one sound-second, and another 
arc drawn, intersecting the x axis at a point 
slightly closer to the gun than 0. This process 
was repeated for each of the trajectory points 
preceding the apparent source for O, always 
keeping the sum of the shell travel time to a 
given trajectory point and the radius of the 
arc from this point in sound-seconds constant. 
In this way a series of arcs was obtained, 
intersecting the x axis in the vicinity of the 
point O. The arcs represented parts of the 
vertical section of the elementary spherical 
waves originating on the trajectory. These 
spherical waves could be considered as inter¬ 
secting the ground plane in circles. The radius 
of each such circle would be the distance along 
the x axis between the intersection of the cor¬ 
responding arc and the projection of its corre¬ 
sponding trajectory point on the x axis. 

In order to keep the construction in one 
plane, the part of the figure below the x axis 
was taken to represent the ground plane, and 
the part above the x axis was taken as the 
vertical plane of the trajectory. In this way the 
circles of intersection of the elementary spheri¬ 
cal waves with the ground surface could be 
drawn below the x axis. This was done for all 
the trajectory points between the gun position 
and the apparent source for the point O. Inside 
the region of audibility an envelope curve could 
be drawn tangent to all these circles. This curve 
represented the instantaneous position for a 
given trajectory of the ballistic wave ground 
trace at the time its vertex reached the point 0. 
This process was repeated for the trajectories 
of other ranges. A resulting template is shown 
in Figure 12. 

The templates were made by compositing a 
printed template in a transparent plastic to 


CONFIDENTIAL 



NEW METHODS AND EQUIPMENT IN SOUND RANGING 


125 


facilitate the fitting of the templates to an 
experimental curve. The method of doing this 
was developed by the Sweeney Lithograph Com¬ 
pany, Belleville, New Jersey, and the E. I. 
duPont de Nemours Company, Arlington, New 
Jersey, in cooperation with Duke University. 
In the finished template the printing is com¬ 
pletely protected from the effects of wear. 

The tables prepared consisted of (1) 
ballistic-burst time interval tables and (2) 
condensed range tables. The former have 
already been referred to. The time interval 
between the arrival of the ballistic wave and 
the shell-burst wave at the reference point w T as 
readily calculated from the data obtained in 
the construction of the ballistic wave ground 
trace. The values of the time intervals were 
tabulated as a function of range for 1,000-yd 
changes in ranges, for a particular value of d, 
and for all the various guns and charges con¬ 
sidered. The condensed range tables were 
prepared to facilitate the calculation of the 
range and deflection effects for the shell 
trajectory when conditions are not standard. 

One hundred sets of templates and tables 
applying to the two German guns (K18 and 
Mrs 18) were delivered to the Field Artillery 
Board for shipment overseas. In addition, 
templates and tables applying to the American 
155-mm M-l gun were delivered to the Field 
Artillery Board for training purposes. 


5 4 NEW METHODS AND EQUIPMENT 
IN SOUND RANGING 

5,41 Obtaining the Data 

Methods 

Coincident with the work described in the 
last chapter on possible methods of improving 
the standard method of acoustic gun ranging, 
the Division of Physical War Research at Duke 
University gave its attention to the possibility 
of developing new methods which might possess 
certain advantages over the standard method. 
In this connection an investigation was made 
of the possibility of (1) ranging by means of 
seismic waves, (2) developing a sound-ranging 
method based on the doppler effect, and (3) 


developing a multiple-short-base method of 
sound ranging. The first two methods proved 
unpromising, and work on them was aban¬ 
doned, but the third method was successfully 
developed and led to the Dodar system, now 
adopted by both the U. S. Army and Marine 
Corps. 

Seismic Method. 7 The study of seismic propa¬ 
gation was undertaken to determine the possi¬ 
bility of detecting and ranging artillery by 
means of earth-borne vibrations. Previous work 
by others 33 had been limited to investigations 
of comparatively deep-traveling refracted and 
reflected seismic waves and had indicated the 
impracticability of using these waves success¬ 
fully for gun ranging. Therefore, the scope of 
this study was limited to the use of waves 
traveling along the surface of the earth. 

The method of utilizing earth-borne vibra¬ 
tions for artillery ranging is fundamentally 
the same as that of using airborne sound, 
differing only in the method of detecting the 
vibrations and in the corrections to be applied. 
Since the medium in this case is stationary, the 
only variable which is considered is the change 
of the velocity as a result of varying earth 
structure between the source and the detection 
devices. 

As a result of field tests at Fort Bragg the 
following conclusions were reached: 

1. The use of earth-borne surface waves 
alone for seismic artillery ranging is not 
feasible because: the artillery-initiated surface 
waves do not possess sufficient energy at the 
usual ranging distances to be detected, and the 
propagation velocity of seismic surface waves 
varies with the terrain over which the ranging 
is to be done—a factor extremely difficult to 
evaluate. 

2. While seismic vibrations in the surface 
layer are initiated by gunfire, these vibrations 
do not reach the geophones through the ground 
at distances normally used in sound ranging. 
They appear to result from and to be of a 
magnitude proportional to the intensity of the 
airborne sound, and they arrive almost simul¬ 
taneously with the airborne sound. 

3. The use of geophones actuated by airborne 
waves is less satisfactory than the use of micro¬ 
phones designed specifically for airborne sound. 


CONFIDENTIAL 



126 


GUN RANGING AND LOCATING SYSTEMS 


Dop'pler-Effect Method. 22 It was suggested 
that the doppler effect be employed to deter¬ 
mine the direction of arrival of a sound from 
a gun. This method was analyzed to determine 
the character of the devices which would be 
used to apply it. 

The doppler effect is most often observed 
as a change in frequency from that emitted at 
the source when there is relative motion be¬ 
tween the source and the observer. However, 
such relative motion may be said to cause a 
change in wavelength of any observed acoustic 
phenomenon, such as the sound impulse emitted 
by a gun when it is fired. Such a change is also 
a doppler effect. In sound ranging on guns the 
only relative motion which can occur must 
necessarily be on the part of the observer. 

When a gun is fired or a shell explodes at the 
end of its trajectory, the resulting acoustic 
wave is highly complex as to frequency. Any 
attempt to extract from it a single frequency 
component for the purpose of applying a 
doppler effect in such manner as to indicate 
the direction of the wave’s origin appears 
highly impracticable. Because of the impulse 
nature of the wave, it would seem that the 
analysis must necessarily be applied to the 
length, on a time base, of some selected and 
easily identified portion of the impulse. Since 
there can be no predetermined and exactly 
known length for this function, a comparison 
of two observations of the same impulse would 
be required. Either one or both of these obser¬ 
vations should be modified by the doppler effect. 
The short duration of the sound would probably 
require some form of recording device, such 
as a multiplicity of identical microphones 
occupying a succession of positions in the sound 
field in some predetermined and systematic 
manner. A short base for these microphones is 
indicated by the requirements for their co¬ 
ordinated movement or switching and by the 
necessity that the signal received by every 
microphone should be identical except for the 
modifying effect of the doppler principle. 

No actual device was constructed, but several 
methods were suggested, such as: (1) two 
microphones on opposite ends of a rotating 
boom, (2) a number of microphones uniformly 
spaced around the circumference of a rotating 


wheel, (3) microphones attached to a moving 
belt, (4) microphones moving with linear 
simple harmonic motion, and (5) stationary 
microphones with a commutation device for 
switching each one in and out at such a rate 
that it would appear as if a single microphone 
were moving with simple harmonic motion. 

Because of the highly transient nature of the 
sound from a shell burst, it does not appear that 
there is any practicable means of detecting 
and analyzing a particular frequency com¬ 
ponent of the transient after this component 
has been modified by the introduction of a 
variable doppler effect. The doppler method, 
therefore, reduces to the measurement of the 



Figure 17. Determination of sound source by 
multiple-short-base method. 

changes in the times of occurrence of certain 
significant and easily identified phenomena in 
records of the wave motion. Since the measure¬ 
ment of time rather than frequency appears to 
be the only solution which the method permits, 
there seems to be no reason to regard the 
doppler method as possessing significant ad¬ 
vantages over the conventional method. 

The application of the doppler method re¬ 
quires the use of very-short-base microphone 
arrays, without in any way overcoming the 


CONFIDENTIAL 






NEW METHODS AND EQUIPMENT IN SOUND RANGING 


127 


inaccuracies and deviations which are known 
to affect the results from such arrays. 

There appears to be no obvious method for 
eliminating from the observations the air dis¬ 
turbances produced if moving, rather than 
commutated, stationary microphones are used. 
There is no significant difference between con¬ 
ventional short-base sound-ranging methods 
and the use of commutated stationary micro¬ 
phones. 

Multiyle-Short-Base Method*’ 9 In view of 
the increased emphasis on mobility and speed 
of operation, an investigation was made of the 
possibility of developing a sound-ranging 
method in which several short bases of two 
microphones each are employed rather than the 
standard long straight base of four to six 
microphones with 2- or 4-sound-second spac- 
ings. 

If two microphones are located on a line of 
predetermined length, it is possible by time 
measurements to determine the position of 
the sound source, as explained in Section 5.2. 
Although only two pairs of microphones are 
required to determine the location of the sound 
source, three pairs are preferable, as the inter¬ 
sections of the three direction lines seldom 
meet in a single point, but generally form a 
triangle, as shown in Figure 17. From this 
triangle the probable location may be estimated 
and some idea gained of the relative inaccura¬ 
cies introduced through variation of “met” 
conditions and terrain. 

Early in 1948 the physical research group of 
the Division of Physical War Research under¬ 
took a series of tests to determine the varia¬ 
tions in the propagation of sound due to atmos¬ 
pheric conditions. 5 ’ 10 These tests are discussed 
in Section 5.5. In general, it was found that the 
observed time variations were independent of 
the frequency of the sound, and that the mean 
time deviation increases with microphone 
separation in a fairly linear relationship for 
small separations, with a leveling-off tendency 
for larger ones. Certain of the data were used 
in evaluating the requirements to be imposed 
on the time interval measuring equipment to be 
used with a short base, and a probability study 
was made showing the degree to which errors 
in location are dependent on time variations. 


The data were of a very preliminary nature and 
could not be used in setting hard and fast re¬ 
quirements. However, it was found that an 
instrument having an accuracy of 1 or 2 milli¬ 
seconds would be satisfactory for use in sound 
ranging up to 5,000 yd. Such a portable, elec¬ 
tronic time interval measuring device was 
developed for use with a short base. This sys¬ 
tem, Dodar, will be more completely described 
in the sections immediately following; it may 
be stated here that tests with the instrument 
showed that it could be used in sound ranging 
over distances up to 8,000 yd and sometimes 
farther. 

Equipment 

Dodar.*’ 9 From its inception the communica¬ 
tions group of the Division of Physical War 
Research at Duke University was concerned 
with the development of new sound-ranging in¬ 
struments which could be used with small 
microphone spacings in contrast with the cur¬ 
rently used Army systems of 2- and 4-sound- 
second spacings. Dodar is such a portable sys¬ 
tem for determining the direction of impulse 
sounds. Two or more of these units operated 
together will determine the direction and range. 
In general, two systems were studied—the Re¬ 
corder Type and the Time Interval Dodar. An 
instrument employing magnetic tape recording 
was developed by the Bell Telephone Labora¬ 
tories under contract with the Signal Corps 
Development Laboratory, Fort Monmouth, New 
Jersey. This instrument was tested and evalu¬ 
ated by the Signal Corps during the latter part 
of 1943, whereupon further work on it by the 
Division was abandoned. The Time Interval 
Dodar, on the other hand, was carried to com¬ 
pletion by the Division and resulted in two 
models, the D-2 and the D-3, the latter being an 
improved model. The fundamental principles 
are the same for both. 

Recorder Type Dodar. This system is based 
on the idea of recording the received signals, 
thereby providing the opportunity of playing 
them back at will and measuring the difference 
in arrival times. The recording is made on a 
magnetic steel tape arranged in an endless loop 
to provide repetition of the signal, thus permit¬ 
ting the operator any amount of time he desires 


CONFIDENTIAL 



128 


GUN RANGING AND LOCATING SYSTEMS 


to obtain a reading. The magnetic tape, how¬ 
ever, does not record and reproduce extremely 
low frequencies satisfactorily. To remedy this 
deficiency, a modulation type of recording was 
proposed such that the carrier frequency would 
be in the center of the optimum frequency 
range for recording and reproducing, thus pro¬ 
viding an adequate frequency spectrum. 

Figure 18 is a block schematic of the Recorder 
Type Dodar showing the required components 
with their functions shown pictorially below. In 


demodulated in accordance with standard tech¬ 
niques. At this point, the output waves are 
replicas of the original air waves, the time 
sequence and differences having been main¬ 
tained. In listening to these two outputs binaur- 
ally, the sound will appear to come from some 
direction in front of the observer. 

The construction of the reproducer is such 
that the reproducing pole pieces and, therefore, 
the sound may be advanced or retarded. Opera¬ 
tion of the phasing control will produce the illu- 



YIELDS SIMULTANEITY 
WITHIN ± 90° 


YIELDS SIMULTANEITY 
WITHIN ± 1° 




Ua- 

T#ri"'" 

—A- 

■•fir—[— 



t 2 


Figure 18 . Recorder Type Dodar. 


A- 


the upper left-hand corner is shown a gun with 
a representation of the sound wave as it pro¬ 
gresses along and passes over microphones 1, 2, 
and 3. The microphone in combination with a 
modulating bridge circuit produces an a-m sig¬ 
nal. The modulated carrier is amplified to pro¬ 
duce the appropriate level for the recorder. The 
remote control shown feeding into the recorder 
is so arranged that when the operator hears the 
signal he is able to stop any further recording 
by a push button. This control automatically 
places the recorder-reproducer in the reproduc¬ 
ing condition. The outputs of the three channels 
are then made available, two at a time, by a pair 
selector switch so that the time difference be¬ 
tween any two signals may be obtained. Each of 
a pair of reproduced signals is amplified and 


sion that the sound moves around in space. The 
control is adjusted to make the sound appear to 
be immediately in front of the observer. When 
this occurs the signals are being applied to the 
ears simultaneously. 0 The wavelets shown im¬ 
mediately below the headphone monitor have 
been drawn as though such an adjustment had 
been made. In other words, approximate simul¬ 
taneity has been achieved. A finer adjustment is 
made with the aid of the phase detector and its 
null-indicating instrument, resulting in simul¬ 
taneity within 1 degree for the frequencies in¬ 
volved. 


c Certain unpublished tests by the Bell Telephone 
Laboratories indicate that such simultaneity may be 
achieved binaurally well within 90 degrees of a 20-c 
wave. 


CONFIDENTIAL 















































NEW METHODS AND EQUIPMENT IN SOUND RANGING 


129 


Such precision would provide time measure¬ 
ments with an accuracy of about 0.1 millisecond. 
It was anticipated that this would permit sound 
ranging up to large ranges (say 5,000 yd), with 
microphone spacings of about 25 ft. It was 
further hoped that the aural perception of the 
operator would enable him to separate and 
identify several signals received in a short time 
period. 

In general, the various components were de¬ 
signed to meet their individual requirements 
without any serious complications. However, de¬ 
signs for two of the components, namely the 
microphone and recorder-reproducer, were 
never satisfactorily evaluated. The type of 
structures used in the microphone design pro¬ 
duced serious variations with temperature, with 
the result that the system could not be stabi¬ 
lized. However, a more serious difficulty was en¬ 
countered: in the reproducing condition, the 
steel tape produced a rather high noise, thereby 
preventing the detection of low-strength sig¬ 
nals. This limited the device to very short 
ranges. 

Time Interval Dodar, D-2. This Dodar model 
was developed for the Marine Corps in connec¬ 
tion with invasion tactics in the South Pacific. 
The Marine Corps set the following require¬ 
ments : 

1 . Portability: all equipment should be light¬ 
weight, rugged, and usable in amphibious opera¬ 
tions. 

2. Personnel and transportation: equipment 
should be limited to a small unit, capable of 
being landed in the early stages of an operation. 

3. Accuracy: the design should give accuracy 
commensurate with counter-battery fire. 

The first two characteristics should take pref¬ 
erence over any increase in accuracy beyond the 
point of counter-battery fire. 

At the beginning it was felt that a direct- 
reading instrument having an inherent ac¬ 
curacy of 1 millisecond could be developed. A 
probability study was made of the errors which 
would be obtained with such an instrument; 
it was found that at a range of 5,000 yd or less, 
an accuracy of 200 yd would be obtained if the 
product of the microphone spacing d and station 
spacing s was set at a value of 10 6 sq ft. The 
study indicated that such an accuracy would be 


obtained at least 80 per cent of the time for 
azimuths not exceeding 50 degrees. 

Studies made by the physical research group 
showed that the mean time deviation due to 
atmospheric fluctuations as observed at two 
microphones would approximate 1 millisecond if 
the spacing were 400 ft. If, therefore, the in¬ 
strument error approximates that introduced 
by atmospheric variations, the design should be 
optimum from the standpoint that both errors 
are equal. It was felt, however, that the error 
caused by these atmospheric variations might 
exceed 1 millisecond, with the result that the 
product sd must be increased to maintain the 
same accuracy. Hence the basic requirement for 
microphone and station spacings was set at 
400 ft for the microphones and 2,000 yd for the 
stations. 

In addition to this arbitrary setting of the 
time accuracy required, the instrument should 
measure time intervals regardless of the direc¬ 
tion from which the sound approaches the mi¬ 
crophone sub-base, and the instrument must 
provide a reading sustained long enough to be 
read by the operator, during which period sub¬ 
sequent signals must not affect the reading. 

Operation of the instrument may be under¬ 
stood by referring to Figure 19, which gives a 
block schematic diagram of the Time Interval 
Dodar; below this diagram the functions of the 
set are shown pictorially. In the upper left-hand 
corner is a picture of a gun with a representa¬ 
tion of the sound wave as it progresses along 
and passes over microphones M R and M L . As 
shown in the block diagram on p. 130, micro¬ 
phone M r produces an electric wave beginning 
at time T lt whereas microphone M L produces an 
electric wave beginning at time T 2 . These waves 
are replicas of the wave which existed in the air, 
and each is amplified to produce a signal large 
enough to operate its associated trigger circuit. 
Operation of the trigger circuits results in two 
voltage outputs which are combined in the 
timing circuit to produce a rectangular pulse 
whose length is determined by the difference in 
times T x and T 2 . This pulse is transformed by 
the timing circuit into a voltage which is “read” 
by the vacuum-tube voltmeter. The meter is 
calibrated to interpret the duration of this pulse 
and hence indicates directly the time difference 


CONFIDENTIAL 



130 


GUN RANGING AND LOCATING SYSTEMS 


T 2 — T l . Transformation of the pulse into the 
meter reading is made with the aid of a re- 
sistance-capacitor-relay circuit which charges 
the capacitor through the resistance for the 
duration of the pulse. Because a capacitor has 
the fundamental property of building up a 
voltage proportional to the time for which a 
constant current flows into it, the voltage ap¬ 
pearing at its terminals will be practically a 
linear function of the time difference, provided 
the capacitor voltage obtained is a small frac¬ 
tion of the charging voltage. 

Shown also in Figure 19 are the blocks repre¬ 
senting the battery box with its associated 6-v 


As a result of field trials, the Marine Corps 
Equipment Board suggested that the Dodars be 
given a trial under actual combat conditions. It 
requested the construction of 25 units, 20 of 
which were to be used by the Marine Corps in 
combat, while the remaining 5 were to be used 
jointly by the Board and the Division of Physi¬ 
cal War Research for further testing. One of 
the five was delivered to the British Ministry 
of Supply Mission, and forwarded by them to 
the Larkhill Laboratories in England for tests 
by the Air Defense Research and Development 
Establishment. 

It was decided to have most of the work done 




WAVE NQ 2 

LV 

1 A 

i 

i 

bn 
+ 0*1 + 

4= 

Y 

II 

I.O + 

* 

METER RD6 


i ^ V/ 

j j 

ITF^ sr iH 



i i ii 

Ti T 2 \ T 2 Tj T 2 

ELECTRIC PULSE AMPLIFIED 

Figure 19. Time Interval Dodar. 


storage battery. All necessary connections for 
heater and high-voltage supply are made 
through a single connecting cable between the 
box and the instrument. 

The D-2 Dodar instrument weighs 26.28 lb 
and is 14% in. long, 11% 6 in. wide, and 10% in. 
high. The meter is calibrated in milliseconds 
with a range of ±300. Special precautions were 
taken to maintain the calibration over a wide 
range of temperature and battery voltage. The 
associated microphones used with this model 
were T-21-B’s, modified as described in Section 
5.3 to give an improved high-frequency response. 
The battery box weighs 16.56 lb with batteries, 
and is 11 7 / 16 in. long, 7% in. wide, and 10% in. 
high. 


by outside agencies, and limit the work of the 
Division to the final assembly, calibration, and 
testing. The Airborne Instruments Laboratory, 
Mineola, New York, did the assembling and 
wiring of the instruments. The final assembly 
included the application of a moistureproofing 
and fungus-resistant finish, in accordance with 
the Signal Corps Specification No. 71-2202 
entitled “Moisture and Fungus Resistant Treat¬ 
ment of Signal Corps Ground Signal Equip¬ 
ment (Over-all Treatment of Assembled Equip¬ 
ment).” Each instrument was calibrated by 
supplying each of its input channels with a 
steep wave front signal separated by known 
time intervals. These pulses were produced by 
a timer mechanism and circuit, capable of 


CONFIDENTIAL 











































NEW METHODS AND EQUIPMENT IN SOUND RANGING 


131 


cyclically producing pairs of pulses spaced at 
time intervals ranging from 0 to 500 millisec¬ 
onds with an accuracy of better than 0.1 milli¬ 
second. 

A series of field tests with the D-2 Dodar 
were carried out, employing both TNT and guns, 
first by members of the Division, and finally by 
the Marine Corps Equipment Board at Quantico, 
Virginia. In general, the tests were designed: 
(1) to determine the effectiveness of the Dodar 
as a sound-ranging instrument, (2) to check 
the probability studies which had been made, 
and (3) to determine the criteria for micro¬ 
phone placement and optimum meteorological 
corrections to be applied. 

The results of the final tests are summarized 
in the following excerpt taken from the Marine 
Corps Equipment Board Report No. 192 of 
March 21, 1944: 

CONCLUSIONS: 

From the results obtained with the new Dodar sets, 
under favorable meteorological conditions and with a 
largely untrained crew, it is felt that the units with 
trained crews will perform in the field with sufficient 
accuracy for counter battery fire. It is believed the size, 
portability, and ruggedness of the sets, together with 
the relatively small operating crew, will fulfill the re¬ 
quirements of the Marine Corps. As shown under 
Results, the accuracy of the location increases with the 
number of readings, especially under gusty-turbulent 
meteorological conditions. Much judgment must be exer¬ 
cised by the Sound Ranging officer in evaluating the 
accuracy of the plots, taking into consideration the 
weather, range, and other necessary factors. 

More specifically, using a 400-ft sub-base, a 
station separation of 2,000 yd, and ranges be¬ 
tween 4,000 and 10,000 yd, the final tests gave: 

83 per cent of the locations within 200 yd; 

74 per cent of the locations within 150 yd; 

44 per cent of the locations within 100 yd; 

22 per cent of the locations within 50 yd. 

Other points brought out by the tests are dis¬ 
cussed under Handling the Data. 

The Dodar was placed in operation by the 
Marine Corps in combat areas with varying 
degrees of success. The most successful per¬ 
formance was in the Iwo Jima campaign, during 
which, according to a report of a combat officer 
to Headquarters, 13th Marines, 5th Marine Di¬ 
vision, dated April 18, 1945, Dodars were used 
and “a total of 54 definite enemy guns and 
mortars were located and 90 additional unveri¬ 


fied plots and directions were relayed to the 
counter battery officer . . . Dodars were found 
very useful during this campaign, despite many 
handicaps.” 

Improved Time Interval Dodar, D-3 (U. S. 
Signal Corps AN/PNS-1). The D-2 model was 
developed primarily for the U. S. Marine Corps 
for use as an experimental gun locator, with a 
range of approximately 5,000 yd. However, the 
Field Artillery Board also made extensive tests 
on this model, which are reported in their test 
number S-49-L Item No. 722-A File No. 413.684. 
This report indicated that the instrument was 
desirable and suggested it be adopted for use 
by Field Artillery Observation Battalions for 
location of mortars and light artillery. The fol¬ 
lowing modifications were recommended: (1) 
improvement of the construction of the set to 
further ensure its watertightness and buoy¬ 
ancy, (2) provision of a frequency-selector con¬ 
trol to adapt the instrument’s frequency re¬ 
sponse to the received signal, (3) provision of a 
light to illuminate the meter dial during dark¬ 
ness, and (4) provision of a power supply to 
replace the dry batteries, if time permitted. 

In addition to these recommendations, the 
field tests on the D-2 and the experience associ¬ 
ated with its manufacture indicated the need 
for additional development. A program was 
prosecuted which resulted in a laboratory model 
incorporating all the features recommended by 
the Field Artillery Board except the vibrator 
power supply, and the following additional im¬ 
provements recommended by members of the 
Division: (1) elimination of tube selection by 
the use of a newly developed relay and improved 
types of tubes, (2) use of a meter with a pre¬ 
calibrated scale, (3) improved resistance to ex¬ 
cessively humid conditions, (4) increased sta¬ 
bility, and (5) increased sensitivity. 

The D-3 Dodar instrument is about the same 
size as the D-2 model, which it somewhat re¬ 
sembles, both models being based on the same 
theory of operation; however, the D-3 Dodar 
incorporates the improvements listed in the last 
section. The major changes in mechanical de¬ 
sign were the use of stainless steel for fabrica¬ 
tion of the instrument and battery box, and a 
change in the panel layout to provide for 
changes in type and location of controls. Figure 


CONFIDENTIAL 



132 


GUN RANGING AND LOCATING SYSTEMS 


20 shows the Dodar instrument and associated 
equipment (cases, batteries, and leads). Figure 

21 gives a closer view of the instrument panel. 
Stainless steel, y 32 in. thick, was used for 

both the instrument and battery box in place 
of the % 2 in. aluminum alloy used in the 
older model. This was done to provide a better 
type of construction and ensure the case’s 
being watertight. No appreciable difference in 
weight resulted from this change. Although this 
case was developed primarily for the Dodar, the 



Figure 20. D-3 Dodar instrument and associated 
equipment. 

design is applicable to many types of portable 
instruments. It satisfies the military require¬ 
ments, namely, that it (1) must be portable, 
(2) must float with the apparatus enclosed, (3) 
must withstand when closed immersion under 
six feet of water, and (4) must be rain- and 
fungus-proof when uncovered for use. 

An important principle used in the design of 
the cases was the method of obtaining a water¬ 
tight seal with a rubber gasket, which involves 
enclosing the rubber so completely that under 
the pressure of the seal it cannot cold flow and 
thereby relieve the desired pressure. It was also 
found necessary to reinforce the cases at all 
points where outside hardware is attached. 17 

After the laboratory model of the new Dodar 
was exhibited to members of the Field Artillery 
Board, it was decided to procure finished models 
of the new unit in the shortest possible time. 
Therefore, Duke University was authorized on 
October 10, 1944, to manufacture units of the 


new model under “crash procurement.” A total 
of 100 units was made and distributed as fol¬ 
lows: 65 units, U. S. Signal Corps (60 of these 
units were for use by the Field Artillery, the 
remaining 5 units for standardization tests by 
the Signal Corps) ; 25 units, U. S. Marine Corps; 
2 units, British Ministry of Supply Mission, on 
a previously unfilled order; 8 units, Duke Uni¬ 
versity. These eight units were for cooperative 
tests by Duke University with the Field Artil¬ 
lery Board and the Marine Corps Equipment 
Board. 

The facilities of Duke University were not 
adequate for producing 100 units. Arrange¬ 
ments were made, therefore, to have the major 
part of the manufacturing, testing, and calibra¬ 
tion done by the Altec Lansing Corporation of 
Hollywood, California. Certain critical parts re¬ 
quiring special testing were purchased by Duke 
University, tested, and shipped to them. Engi¬ 
neers from the Division visited Altec Lansing 
to assist in the development of the testing pro¬ 
cedures and to train their engineers in the 
method of factory calibration. A time interval 
generator developed by the Division was fur¬ 
nished for use in calibrating the Dodars. 

The final acceptance tests on the new Dodar 
were started by the Field Artillery Board at 





dBh 


Figure 21. D-3 Dodar instrument panel. 

Fort Bragg and completed by the Field Artil¬ 
lery School at Fort Sill, Oklahoma. These tests 
determined the ability of the Dodar to function 
as a sound-ranging instrument from the stand- 


CONFIDENTIAL 








NEW METHODS AND EQUIPMENT IN SOUND RANGING 


133 


point of its durability, accuracy, range, and 
tactical use. The results of these tests indicated 
that the accuracy of the Dodar was satisfactory, 
that it was adequately protected for military 
use, and that it was capable of locating field 
artillery pieces, mortars, machine guns, and 
small arms fire. The accuracies of all verified 
sound-ranging locations made in these tests are 
shown in Table 2. 


Table 2 


Error in 

Per cent of 

location 

total locations 

Less than 50 yd 

26 

Between 50 and 100 yd 

32 

Between 100 and 150 yd 

19 

Over 150 yd 

23 

A total of 53 locations was made during the 
two periods of field testing from approximately 

300 recorded sounds from 

these locations. 

Weapons successfully located under favorable 
conditions, with maximum distances (measured 
from an inner sub-base) at which locations were 
made, are shown in Table 3. These are probably 
not the greatest possible ranges under most 

favorable conditions. 


Table 3 


Weapon 

Distance (yd) 

60-mm Mortar 

3,000 

81-mm Mortar 

5,000 

42-in. Mortar 

4,500 

Battery of .30-cal machine guns 4,500 

.50-eal machine guns 

4,500 

Rifles and carbines 

2,000 

Anti-tank rocket launcher 

4,100 


Time did not permit extensive combat use of 
the D-3 Dodar before the end of World War II. 
Although used in the Okinawa campaign it was 
not reported on very favorably. However, it 
is felt by members of the Division and by 
Marine Corps personnel most familiar with the 
Dodar that it was not given a fair test on 
Okinawa and that it is capable of a much better 
showing. 

Ultralightweight Dodar System. In the inter¬ 
ests of reduced size and weight, as well as re¬ 
duced susceptibility to damage from water, dirt, 
etc., a new lightweight crystal microphone was 
developed by the Division. This microphone is 
described in the next section. At the same time 


a breadboard model of a vibrator power supply 
which could be mounted in the battery box was 
developed. The weight of this box with the 
vibrator power supply is less than with the 
original four “B” batteries, and the new micro¬ 
phone weighs 4 lb as against 24 lb for the 
T-21-B. A demonstration of a complete Dodar 
system employing the improved D-3 instrument, 
the new crystal microphone, and the model of 
a power pack was conducted by members of the 
Division for representatives of the Marine Corps 
Equipment Board and the Marine Corps Field 
Artillery School at Quantico, Virginia. The re¬ 
sults of this test indicated that the overall 
sensitivity of the new microphone (designated 
the T-l) and Dodar was greater than the old 
combination of the T-21-B modified microphone 
and Dodar. When the proper frequency band 
was selected, there was a substantial improve¬ 
ment in signal-to-noise ratio. Thus it was pos¬ 
sible to range on guns located beyond 5,000 yd 
in the presence of noise due to airplanes, trucks, 
and wind which would have made sound rang¬ 
ing impossible with the T-21-B and Dodar setup. 
It was also noted that there was less necessity 
of checking the field calibration of the Dodar 
when the vibrator power supply was used. Ter¬ 
mination of the Division’s contract prevented 
further development of this system. 

New Lighticeight Crystal Microphone 11 ’ 21 
The principal energy contained in sound waves 
from field artillery guns and howitzers lies in 
the region from 5 to 40 c. Substantial compo¬ 
nents, especially in the case of the smaller field 
weapons, may reach values as high as 100 c. 
Sound ranging has, therefore, required the 
design and use of highly specialized micro¬ 
phones particularly suited to the efficient re¬ 
ception of frequencies within this region. 

The reduction of the effects of wind and other 
extraneous sounds, which frequently cause 
severe disturbance in the upper portion of this 
frequency band, has led to the use of relatively 
sharp cutoff, low-pass filters in microphones. 
Filters for such low frequencies, whether elec¬ 
tric or acoustic, have in turn resulted in large, 
heavy microphones which are quite unsuitable 
for highly portable use. 

The need for a reliable, small, lightweight 
microphone for sound ranging became apparent 


CONFIDENTIAL 











134 


GUN RANGING AND LOCATING SYSTEMS 


during the development of Dodar (AN/PNS-1). 
The choice of microphone available for use with 
Dodar was limited to two types normally uti¬ 
lized for general sound ranging but each of these 
has serious disadvantages. 

The T-21-B condenser microphone has been 
the standard Service model since 1941. It is 
large, heavy, and very cumbersome to carry 
(see Figure 22). Because its case is a part of 
the electric circuit, the microphone must be 



Figure 22. Comparison of T-l microphone ( left ) 
and T-21-B modified microphone {right). 

enclosed in a heavy rubber boot when in use. 
In addition to its physical inconvenience, the 
T-21-B is subject to failure through entry of 
foreign material. This fault was minimized but 
not avoided in the modification described in 
Section 5.3. 

The T-23 hot wire microphone, which is some¬ 
what smaller and significantly lighter than the 
T-21-B, is the newest Signal Corps type and 
only recently was adopted as the standard re¬ 
placement. Production delays, however, made it 


unavailable until after World War II. In addition 
to its unavailability in large numbers, its use 
with Dodar was prohibited by its extremely 
short filament battery life. Both types of micro¬ 
phones are subject to serious damage from tem¬ 
porary submersion, which is a frequent occur¬ 
rence in field operations. 

In addition to being designed for Dodar in 
particular, it was anticipated that a new, highly 
portable microphone might be designed to con¬ 
tain features which would make it superior to 
other microphones for general sound ranging. 
Such characteristics as ability to withstand 
temporary submersion, a convenient means of 
selecting several upper frequency cutoffs to suit 
particular sound-ranging conditions, and ease of 
maintenance without exposing vital parts are 
desirable for all sound-ranging purposes. 

The following requirements were considered. 

1. The highly portable microphone should be 
as small as possible—preferably of a size per¬ 
mitting its being carried by pocket. 

2. It should be as light as possible—prefer¬ 
ably under 5 lb. 

3. It should be capable of operation with its 
case in contact with the ground. 

4. Its filament battery life should be in¬ 
creased as much as possible over present types. 

5. The highly portable microphone should be 
capable of use in circuits designed for T-21-B 
and T-23 microphones. 

6. It should have approximately the same 
sensitivity and power output capacity as the 
T-21-B microphone. 

7. It should have conveniently selectable, 
upper frequency cutoffs permitting its use for 
ranging on all sizes of field artillery weapons 
with the maximum elimination of extraneous 
noise. The frequency ranges set ran from 3 to 
40 c, from 3 to 60 c, and from 3 to 80 c. 

8. The microphone must be capable of satis¬ 
factory operation over a temperature range of 
from 0 to 120 F and should not be damaged 
by extended exposure to temperatures of 160 F. 
The effects of humidity on the microphone 
should be minimized. 

Two models of the highly portable micro¬ 
phone were developed: an experimental model 
T-l and a model T-2 for combat use with Dodar. 

Five T-l microphones were constructed and 


CONFIDENTIAL 










NEW METHODS AND EQUIPMENT IN SOUND RANGING 


135 


subjected to extensive laboratory and field tests. 
They were then turned over to the Camp Evans 
Signal Laboratory of the Signal Corps and to 
the Field Artillery Board. 

At the conclusion of these acceptance tests 
the improvements anticipated from the Division 
tests and those resulting from the Service ac- 



Figure 23. T-l Crystal microphone—analogous 
circuit of transducer and acoustic filter. 


Mi = Acoustic inertance of filter plug 
r a = Acoustic resistance of filter plug 
Ca = Acoustic capacitance of filter chamber 
mi = Mass of diaphragm 
Cmi = Compliance of diaphragm 

Cm 2 = Compliance added to diaphragm by acoustic backplate 
m 2 = Mass added to diaphragm by acoustic perforations in 
backplate 

rjv/i = Resistance added to diaphragm by acoustic perfora¬ 
tions on backplate 
Cmz = Compliance of link 
m 3 = Mass of link and nosepiece 
ra 4 = Effective mass of mounted crystal 
CMi = Compliance of crystal 
Cms = Compliance of crystal mount. 

ceptance tests were combined in the design of 
a combat model, designated Type T-2. Based on 
the performance of the Type T-l microphone 
and the modifications, the T-2 model was ac¬ 
cepted for use with Dodar (AN/PNS-1) and 
was given the Signal Corps nomenclature 
M-12 ( )/TN. 

Ten Type T-2 microphones were constructed, 
submitted to tests by the Division, and turned 
over to the Armed Forces prior to the close of 
the contract. Information pertinent to manu¬ 
facturing specifications and a complete set of 
manufacturing drawings of this model were 
submitted to the Camp Evans Signal Labora¬ 
tory. 

A further modified model of this microphone, 
taking into account its use with current stand¬ 


ard and with anticipated sound-ranging appa¬ 
ratus, was authorized as Signal Corps Type 
M-13 ( )/TN. The closing date of the contract 
did not permit the completion of this model. 

The T-l and T-2 microphones are of a crystal 
type, utilizing a recently developed piezoelectric 
crystal of ammonium dihydrogen phosphate. 
This crystal is capable of withstanding tem¬ 
peratures up to 212 F without loss of its piezo¬ 
electric properties, as compared with 130 F for 
the best previous types suitable for use in 
sound-actuated microphones. 

The crystal is contained in a small metal en¬ 
closure (see Figures 23 and 24) which serves to 
protect it from damage due to handling. The 
crystal is cantilever mounted, its free end being 
actuated by a link bar which attaches it to the 
center of a formed aluminum diaphragm (0.002 
in. thick). The crystal, or under, side of the 
diaphragm is protected by a backplate spaced 
0.030 in. from the diaphragm, providing acous¬ 
tic damping for the system and serving to 
support the diaphragm should water invade the 
upper acoustic chamber. 

The microphone contains a two-stage re¬ 
sistance-coupled amplifier (see Figure 24). Ma¬ 
terial reduction in size was accomplished by the 
use of miniature tubes. 

The frequency response of the T-l micro¬ 
phone is obtained by means of two types of 
low-pass filters: (1) an acoustic filter and (2) 
an amplifier interstage filter. The latter is ad¬ 
justable in three steps by means of a simple 
terminal switch, making available three sepa¬ 
rate frequency ranges. 

The physical dimensions of the T-2 micro¬ 
phone (10 in. high with a 4%-in. diameter) and 
its operating weight of 4 lb enable it to be 
carried by pocket or hand as circumstances dic¬ 
tate. Many features were incorporated in its 
design to ensure reliable operation and con¬ 
venient maintenance under field conditions. 

The new lightweight crystal microphone sat¬ 
isfies all the requirements and has a much re¬ 
duced susceptibility to moisture, dirt, etc. Tests 
indicated that no serious damage results from 
complete submersion for at least 12 hours in 
depths experienced in normal field operations. 
Convenient maintenance is possible without ex¬ 
posing any of the critical parts of the micro- 


CONFIDENTIAL 

















136 


GUN RANGING AND LOCATING SYSTEMS 


phone. The overall frequency response of the 
new microphone for each of the three ranges is 
shown in Figure 25. For the microphone alone, 
the ranges (10 db cutoff) were found to be: 
Range H, 4-80 c; Range N, 4-60 c; Range L, 
3-40 c. The recommended use of the ranges is 
as follows: 

Range H—for ranging on small field weapons, 
such as mortars and light artillery, where ex¬ 
traneous noise permits; 

Range N—for general sound ranging; 

Range L—for ranging under conditions of 
extreme extraneous noise when the signal has 


additional improvements in the design were 
recommended by the Division, but time did not 
permit further development. 


5 4 2 Handling the Data 

Systems of Coordination 

In the multiple-short-base method of sound 
ranging for which the Dodar system was de¬ 
veloped, consideration must be given to the 
best method of coordinating the sub-bases em¬ 
ployed (usually three). It is important that this 



Figure 24. T-2 Microphone. 


sufficient energy at very low frequencies to give 
a usable output. 

Field response tests proved the advantage of 
the high-frequency range H for ranging on 
small weapons and for distinguishing the ballis¬ 
tic wave from the muzzle wave. Figure 26 is an 
actual record of a 60-mm mortar, in which the 
middle trace (third from bottom) shows the 
response of the T-l microphone on Range H. 
The response to the low frequencies produced 
by large weapons was found equally good as 
that of the T-21-B or T-23 microphone. In these 
tests the new microphone compared favorably 
with the older types from the standpoint of 
extraneous noise and ground vibration. Further 


coordination be good to ensure that each Dodar 
set is ranging on the same gun at the same time 
and to reduce the time required for the sub¬ 
sequent plotting and relaying of the determined 
plot to the counter-battery officer. There are 
two systems of coordination possible: (1) to 
locate each Dodar instrument near its corre¬ 
sponding microphones and to connect the opera¬ 
tor of the instruments with sound central by 
wire or radio and (2) to locate all the Dodar 
instruments at sound central. 

There are advantages and disadvantages to 
each of the two systems listed. In the first sys¬ 
tem, the observer is nearer the microphones 
should they require repair; if in front of them, 


CONFIDENTIAL 



NEW METHODS AND EQUIPMENT IN SOUND RANGING 


137 


he may be able to pick out the desired gun pulse 
by ear, but both the observer and instrument 
may be in a more exposed position. In the second 
system, observers and equipment may be more 
protected and results obtained from each Dodar 
set may be more readily compared and plotted; 
but longer lines between microphones and in¬ 
struments increase the probability of a set be¬ 
coming inoperative through a line being broken 
or microphone damaged, and repair of such dam¬ 
age would require more time. It was concluded 
by the Division that the choice of system could 
best be decided by actual combat experience, and 



Figure 25. Overall frequency-response curves for 
T-l microphone. 


the matter was left up to the Marine Corps. At 
the end of World War II a conclusive answer to 
the problem had not been received. 

Probability of Gun-Locating Errors 8 

Analytical Study . An analytical method was 
developed for evaluating the probability of lo¬ 
cating a target when two direction lines with 
their angular errors and the spacing between 
their origins are known. The method is based on 
probability and geometrical principles. It was 
found that there is a probability p that the loca¬ 
tion will lie within an ellipse whose major axis 
gives the value of the range error and whose 
minor axis gives the bearing or line error. The 
range error SR is large compared with the line 
error SL and is therefore the only one which 
need be considered in determining the location 
accuracy. The expression for SR is 

6R = 0.775 

sd cos 2 6 


where 

V H = velocity of sound, 

5(A£) = error in arrival time interval, 

R = range, 

s = distance between centers of the two sub-bases, 

d = length of each sub-base, 

6 = azimuth of source measured from normal to 
base, 

h = log e 

V 

It will be noted that SR is directly propor¬ 
tional to R 2 and inversely proportional to the 
product sd. The accuracy is best for small azi¬ 
muths. Figure 27 shows the error in range as a 
function of azimuth for a time interval error of 
2 milliseconds and an sd product of 2.4 X 10 6 
sq ft, values which approximate field conditions 
for the Dodar. 

In preliminary field tests of the Dodar con¬ 
ducted at Quantico, Virginia, a microphone 
base of 400 ft and a separation of 2,400 ft were 
used, giving an sd product of about 10 6 sq ft. 
Calculated and observed errors agreed well 
even though the number of shots was small. 
The proportionality between SR and R 2 was 
found to hold. In the final field trials of the 
Dodar the sd product was increased to about 
2.4 X 10 6 sq ft by increasing the station separa¬ 
tion to about 2,000 yd. The results have been 
summarized previously. The variation of prob¬ 
ability with range error is consistent with 
theory. In conclusion it may be noted that the 
Army, in its field manual (FM 6-120) covering 
the Dodar, recommended that the distance be¬ 
tween flank sub-bases be about half the distance 
from the base to the target area, making the 
angle subtended by the outside microphone sets 
about 500 mils (6,400 mils equal 360 degrees). 

Reduction of Errors Due to Meteorology 
and Terrain 8 ’ 10 

In sound ranging with Dodar, corrections 
must be applied for temperature and wind; the 
effects of terrain must also be considered. 
Furthermore, fluctuations in meteorological 
conditions will affect the time intervals to be 
measured in a random way for which it is hard 
to correct. When smaller microphone separa¬ 
tions with resulting smaller time intervals are 
used, the effect of meteorological fluctuations 


CONFIDENTIAL 







































138 


GUN RANGING AND LOCATING SYSTEMS 


is relatively more important. Conversely, a 
Dodar system may be used to study such varia¬ 
tions. 

Temperature. The effective temperature 
taken by the Army for Dodar sound ranging is 
the same as that for long-base sound ranging 
(see Section 5.2). The Marine Corps uses the 
surface temperature. 

Wind. In the Dodar field tests, a study was 
made of the effect of wind. It was found that a 
simplified wind correction could be used for the 
smaller ranges covered by the Dodar instead of 


Microphones should also be kept away from the 
edge of a wooded section. The best location for 
the microphones was found to be in holes on 
the crest or forward slope of a hill. 

“Met” Fluctuations. It is obvious that any 
system of sound ranging based on airborne 
signals will be adversely affected by motion of 
the transmitting medium or local variations in 
the transmission properties of the sound path. 
Corrections may be applied to neutralize errors 
produced by constant meteorological conditions, 
but variations in the meteorology have not as 



Figure 26. Records of actual guns using various types of microphones. 


the more complex “standard” correction used 
by the Army. It was found that the optimum 
wind correction is given by the 45-second ob¬ 
servations on a rising balloon in the wind. The 
Army has adopted the first-minute wind as the 
effective value, while the Marine Corps has 
chosen to use the 30-second wind observation 
for ranges less than 7,000 yd, the one-minute 
observation for ranges over 10,000 yd, and the 
correction yielding the smallest triangle of 
error for intermediate ranges. 

Terrain. The effect of the terrain in which 
the microphones were located was found to be 
an important factor, but one hard to correct for. 
To minimize this effect the following rule was 
laid down: both microphones of each Dodar 
sub-base should be located in similar terrain. 


yet been mastered. This is a serious weakness in 
gun ranging by sound. In this connection some 
hopeful beginnings were made. The physical re¬ 
search group observed that a relationship 
appears to exist between variations in the 
amplitude of the signals received by two micro¬ 
phones and the associated time differences. The 
value of such a relationship lies in the fact that 
its use permits corrections to be made for the 
variations in meteorological conditions without 
the onerous, if not impossible, measurement of 
the micrometeorological conditions. Hence, a 
peak-reading voltmeter was developed to meas¬ 
ure amplitudes received during Dodar opera¬ 
tion. The resulting tests will be discussed in the 
next section, but it may be stated here that a 
high degree of correlation between variations 


CONFIDENTIAL 









































































































































































































































































































































































































































































































































































SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


139 


in amplitude and arrival time of a sound im¬ 
pulse at a given microphone appears to exist, 
and that a material improvement in sound¬ 
ranging accuracy may be obtained by an appli¬ 
cation of this correlation. 


55 SOUND TRANSMISSION THROUGH 
THE ATMOSPHERE 2 * 5 * 10 

The physics of the propagation of sound en¬ 
ergy through the atmosphere and along a 
boundary, such as the ground, is fundamental 
to the problem of correcting for acoustic gun¬ 
ranging errors, particularly those due to “met” 
and terrain conditions. 



Figure 27. Probable range error vs azimuth. 


In spite of the fact that this study constituted 
a pure research problem of rather long dura¬ 
tion, a section of the Division was assigned to 
the problem. The reasons were twofold: (1) the 
only method of improving sound ranging, 
should certain empirical methods fail, might be 
expected to come through the complete under¬ 
standing of the physics of sound propagation; 
and (2) it was believed that a fundamental 
knowledge of sound propagation would be of 
real value, particularly if World War II lasted 
a long time. 

Two lines of procedure were planned for the 
solution of the problem. The first was an em¬ 
pirical study of the possible relationships ex¬ 
isting between errors in gun ranging and such 
meteorological measurements as could be made. 
The second was a basic physical study, both ex¬ 


perimental and theoretical, of sound transmis¬ 
sion through the atmosphere. At times the 
order of procedure and the nature of the experi¬ 
ments to be performed were decided on the 
basis of their possible value to sound ranging 
rather than on their value to the fundamental 
research. Every attempt was made to reduce 
this interference with pure research to a mini¬ 
mum. 

Considerable exploratory experimental work 
was done. Several lines of attack were followed, 
a considerable amount of data was amassed, 
and some theories to account for these facts 
evolved; however, the problem is far from com¬ 
pletely solved. In some respects the work ap¬ 
pears to be merely a collection of disconnected 
experiments, but it is believed that the re¬ 
sultant experimental data constitute a good 
starting point for further research, and they 
are, therefore, presented in spite of the fact 
that the work was discontinued before their 
full significance could be determined. 


5 51 General Background 

Types of Errors in Sound Ranging. In locat¬ 
ing a gun by sound, consideration must be given 
to the possible sound rays through the atmos¬ 
phere from the gun to the receiving micro¬ 
phones and to the velocity of sound along such 
rays. Furthermore, the sound rays in this case 
are generally confined to relatively low altitudes 
(sometimes practically to grazing incidence) ; 
hence the effect of the presence of a boundary, 
such as the ground, must also be considered. 
Errors in sound ranging fall into two classes: 
instrumental errors and propagation errors. 
The techniques of and the equipment for acous¬ 
tic gun ranging are sufficiently advanced so that 
the instrumental errors are smaller than the 
propagation errors, except on those few days 
when ideal conditions exist, when the latter 
decrease to approximately the same order of 
magnitude as the instrumental errors. There¬ 
fore, the propagation errors, mainly due to 
meteorology, become the most important errors 
to eliminate. 

Macromet vs Micromet. The errors in sound 
ranging due to wind and temperature have been 


CONFIDENTIAL 









140 


GUN RANGING AND LOCATING SYSTEMS 


found to be of two types. First, there are those 
errors due to the overall and slowly changing 
meteorological conditions of the atmosphere, 
hereafter referred to as the macromet, or just 
the “met/’ Second, there are those errors due 
to those meteorological factors which vary 
rapidly and in a random manner with regard 
to both space and time, hereafter referred to 
as the micromet. 

Corrugations. The problem may be stated in 
a slightly different way. If the atmosphere pos¬ 
sessed horizontal homogeneity, and wind and 
temperature gradients were not changing with 
time, the acoustic wave fronts traveling out 
from a sound source would be regular in form, 
becoming approximately a plane wave at rela¬ 
tively great distances from the source. How¬ 
ever, if meteorological or terrain conditions 
vary in an irregular manner from place to place, 
or if wind and temperature fluctuate rapidly 
with time, then one would expect the acoustic 
wave fronts to possess an irregular shape. The 
resulting irregularities are sometimes referred 
to as corrugations or serrations. Such corruga¬ 
tions would, of course, affect the arrival time 
at a given microphone of the sound impulse 
from a gun, or the difference in arrival times 
measured for a pair of microphones. 

Correcting for the “Met.” The existing meth¬ 
ods of correcting for the effect of meteorologi¬ 
cal conditions (Section 5.3) assume in every 
case that conditions in the atmosphere are 
practically steady, that is, that wind and tem¬ 
perature do not change appreciably in the in¬ 
terval between the taking of meteorological 
observations and the application of the correc¬ 
tions computed from them. Furthermore, it is 
assumed that conditions, characterized by a 
single set of vertical soundings, are the same 
for all the microphones of the base. For this 
reason these methods may be said to attempt 
to correct only for the macromet. In practice 
the hope is either expressed or implied that this 
is good enough. However, on the basis of a 
study of the effectiveness of the standard 
method and subsequent work performed by the 
Division, this appears to be too optimistic. In 
undertaking this project it was felt that it was 
necessary to determine: first, how important 
micrometeorological factors are in sound rang¬ 


ing; and second, if such factors are important, 
whether any method of correcting for them 
could be found. 


552 Experimental Procedure 

After a considerable study of microphone 
arrays and consultations with members of the 
Field Artillery Board, Fort Bragg, North Caro¬ 
lina, and the Marine Corps Equipment Board, 
Quantico, Virginia, two types were chosen for 
study by the Division of Physical War Re¬ 
search. The first was the Army straight-base 
system. The second was the employment of 
two or more short-base direction finders, suit¬ 
ably coordinated (the Dodar system). 

A statistical study of the effectiveness of the 
standard method of applying a meteorological 
correction when using the straight-base system 
was made. In this study data were acquired 
under field conditions at Fort Bragg, North 
Carolina, and at Fort Sill, Oklahoma. 

Work on the reduction of errors due to the 
macromet included the following: (1) a com¬ 
parison of various types of microphone bases, 
(2) an investigation of the possibility of im¬ 
proving the methods of evaluating the macro- 
met data, (3) a comparison of the various meth¬ 
ods of correcting for the macromet, and (4) a 
proposed method of sound ranging eliminating 
meteorological corrections. 

The investigation of the effect of wind and 
temperature fluctuations in the atmosphere on 
sound propagation formed a large part of this 
work. The method employed sources of both 
steady, single-frequency sounds and sound im¬ 
pulses. With single-frequency sounds, ampli¬ 
tudes and relative phases were measured at 
pairs of microphone receivers, while with the 
impulse sounds, amplitudes and arrival times 
were measured at the microphones. 

Although impulse sound measurements pro¬ 
vide the most direct approach, it was deemed 
advisable to carry on a parallel study of the 
transmission of continuous sound from a single¬ 
frequency, sine wave source for the following 
reasons. 

1. Gun sounds are made up of a complex 
combination of frequencies, which vary with 


CONFIDENTIAL 



SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


141 


the caliber and charge of the gun, and also from 
gun to gun and from one round to another. 
Analyzing data on impulse sounds is consider¬ 
ably more difficult than analyzing similar data 
on steady-state sounds. 

2. The number of readings in a given time 
period is limited by the rapidity of loading and 
firing a gun, or of placing explosive charges. 
It is sometimes desirable to get closer grained 
sampling with respect to time. With phase 
measurements of sustained, single-frequency 
sound, these readings may be made continuously 
at intervals corresponding to the period of the 
signal. 

3. Gunfire and explosive charges are rela¬ 
tively expensive, requiring special equipment 
and trained personnel not readily available for 
a continuous, extended study at different loca¬ 
tions. 

Measurements with Single-Frequency 
Sources. Two microphones were placed at vari¬ 
ous fixed positions in front of a continuous 
sound source emitting a single frequency. The 
amplitude and phase of the sound at each micro¬ 
phone were recorded either continuously or at 
regular intervals of a few seconds. If the two 
microphones are equidistant from the sound 
source, then, in the absence of wind, horizontal 
inhomogeneities, or unsteady conditions, one 
would expect the amplitude and phase at the 
two microphones to be the same. Presence of a 
steady wind or irregularities of terrain should 
produce a steady difference in phase at the two 
microphones; whereas if the wind is gusty or 
the atmosphere possesses pockets of hot or 
cold air carried by the wind, one would expect 
momentary fluctuations in the phase difference 
as well as in the amplitude at each microphone. 
Actually, the phase difference and the two 
amplitudes were observed to fluctuate continu¬ 
ously. During these experiments various mete¬ 
orological elements such as wind speed, wind 
direction, and temperature were simultaneously 
recorded. 

The fact that the fluctuations appeared to 
follow a normal random distribution suggested 
that it might be advantageous to apply statisti¬ 
cal methods. Since a phase difference between 
two microphones is equivalent to a difference in 
arrival times for a given portion of the wave 


front, it is similarly correctable with a differ¬ 
ence in arrival times for impulse sounds such 
as are encountered in sound ranging. Hence 
fluctuations in phase difference and dependence 
of mean phase difference on various factors 
were studied in these experiments for the pur¬ 
pose of estimating and possibly reducing errors 
in sound ranging due to micrometeorological 
factors. To this end experiments were per¬ 
formed under a variety of conditions. The effect 
of a variation in one condition at a time was 
investigated, so far as possible, as follows: (1) 
changing the source from a point on the per¬ 
pendicular bisector of the microphone base (the 
usual position) to a point in line with the micro¬ 
phone base, (2) varying the separation of the 
microphones, (3) varying the distance between 
the microphones and the source, (4) varying the 
frequency of the source, (5) varying the time of 
day of the experiment, (6) choosing times when 
wind speeds and gustiness were as desired, (7) 
choosing times when wind gradients were as 
desired, and (8) choosing times when tempera¬ 
ture gradients were as desired. It was fre¬ 
quently impossible to get the conditions of (6), 
(7), and (8) as desired. 

Although the microphones in the above ex¬ 
periments were kept in a horizontal line on the 
ground, it became evident in studying the effect 
of wind and temperature gradients that the 
investigation would not be complete without 
including experiments in which one microphone 
was raised vertically above the other. For this 
purpose a 50-ft mast was employed; measure¬ 
ments were made of amplitude and phase as a 
function of the height above the ground and 
of the relation of this structure to (1) wind 
gradient, (2) temperature gradient, and (3) 
nature of the terrain. 

Measurements with Impulse Sources. The 
results of the experiments are summarized 
later; it may be said here that a much better 
understanding of wave front corrugations, of 
micrometeorology, and of sound propagation 
along a boundary was obtained. However, in 
order to apply these results to the problem of 
reducing short-period errors in gun ranging, it 
was felt advisable to relate these experiments 
to others in which impulse sources of sound 
approximating gunfire and actual sound-rang- 


CONFIDENTIAL 



142 


GUN RANGING AND LOCATING SYSTEMS 


ing bases and recording equipment were used. 

It should be stressed that short-period fluctu¬ 
ations in the atmosphere make it useless, except 
under the most favorable conditions, to read 
arrival times to closer than a few milliseconds, 
and that the resulting error becomes much 
more important when a short base is used, for 
then the time difference itself is small. This ap¬ 
plies in particular to the Dodar, where the 
length of the base is usually between V 3 and V 2 
sound-second. Therefore experiments were per¬ 
formed in which Dodars as well as the standard 
GR-3-C equipment were used. 

The various “shoots” during which sets of 
data were obtained may be summarized as fol- 


lows. 



Recording 

Date 

Place 

Source 

Equipment 

5-20-43 

Fort Bragg 

TNT 

GR-3-C 

7-23-43 

Fort Bragg 

240-mm Howitzer 

GR-3-C 

7-29-43 

Fort Sill 

105-mm Howitzer 

GR-3-C 

5-29-44 

Fort Bragg 

155-mm M-l Gun 

GR-3-C and 
Dodars 

5-30-44 

Fort Bragg 

155-mm M-l Gun 

GR-3-C and 
Dodars 

7-10-44 

Quantico 

TNT and 
dynamite 

Dodars 

7-19-44 

Quantico 

TNT and 
dynamite 

Dodars 

1-8- 45 

Fort Bragg 

TNT and mortars 

GR-3-C ana 
Dodars 

1-10-45 

Fort Bragg 

TNT and mortars 

GR-3-C and 
Dodars 


During these experiments, as in those with 
a single-frequency source, the effects of varia¬ 
tions in the following were studied: direction 
of microphone base, microphone separation, dis¬ 
tance of source, meteorological conditions, and 
nature of the terrain. 

Comparison of Impulse and Single-Frequency 
Measurements. Certain characteristics peculiar 
to single-frequency measurements required care 
to be used in comparing the results of the 
single-frequency and impulse measurements. 

There may be a difference under certain con¬ 
ditions between phase velocity, measured in the 
single-frequency studies, and signal velocity, 
given by impulse measurements. Because of in¬ 
terference phenomena, multiple arrivals, and 
other factors, these records may not be inter¬ 
changeable. 

It has not been possible to obtain a practical 
high-intensity source of sustained single-fre¬ 


quency sound in the low-frequency range (5 to 
50 c) in which important parts of the energy 
of gun sounds exist. It is thus necessary to 
extrapolate over a rather wide frequency range. 
The lowest frequency used in the single-fre¬ 
quency studies was 50 c, and most of the work 
was done between 100 and 400 c. 

Even at higher frequencies it is impossible 
to secure a sustained source of sound with peak 
pressures comparable with those encountered in 
gun sounds. 

A two-port siren designed to produce 2.5 kw 
was used in some of the studies described in this 
report, at a maximum distance of 1,600 yd. 
Sound ranging in military operations is usually 
conducted over ranges from 3,000 to 30,000 yd. 
Impulse measurements were made, however, in 
connection with this project as close as 1,200 
yd, resulting in some overlap. 

It should be borne in mind that under some 
conditions the sound path traversed in long¬ 
distance transmission may be considerably dif¬ 
ferent from that encountered in shorter ranges. 


5.5.s Experimental Equipment 

Single-Frequency Measurements 

Several methods of measuring phase and am¬ 
plitude of a single-frequency source were used. 
The most recent and complete system is shown 
in simplified form in the block schematic (Fig¬ 
ure 28). 

The slow-speed oscillograph showed records 
directly on waxed paper, and gave plots of phase 
change and amplitude at a number of micro¬ 
phones as a function of time. The equipment 
contained more elements than are shown in 
Figure 28. A complete duplicate of the equip¬ 
ment shown could be set up, giving the ampli¬ 
tudes at four microphones and three phase 
differences, representing any desired pairs of 
signal inputs (one input, if desired, could be 
from a common reference microphone or the 
input to the speaker). 

Sound Sources. Two loudspeakers were used: 
(1) a two-section re-entrant horn with an open¬ 
ing about 5 ft square and a cutoff frequency of 
50 c and (2) a stereophonic horn system loaned 


CONFIDENTIAL 



SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


143 


to the Division through the courtesy of the 
Bell Telephone Laboratories. 

The 5-ft horn gave an acoustic output of 85 db 
above 10 16 watts per sq cm at 100 yd, using a 
frequency of 200 c. The larger horn gave an 
output 8 to 10 db higher at this same frequency. 
With both systems type 200-DR Hewlett Pack¬ 
ard oscillators were used. 

Two sirens, developed by the Bell Telephone 
Laboratories and built by the Chrysler Corpora¬ 
tion, were used during these studies. One was 
driven by a 10-hp gasoline engine, and the other 
by a 10-hp electric motor powered by a 10-kw 
diesel-driven generator. The siren provided a 
signal level for equal distances approximately 
26 db higher than that available with the 5-ft 
horn, but introduced problems caused by poorer 
frequency stability, greater complexity of wave 
form, and less flexibility of frequency choice. 

Microphones. Conventional microphones were 
used, with dynamic types preferred on the 
grounds of ruggedness and relatively perma¬ 
nent calibration in field use. Most of the 
measurements were with Western Electric type 
633-A transmitters, although WE type 630 
were also used. Wind screens, 12 in. in diameter, 
of approximately hemispherical design, and 
covered with a double thickness of cheese-cloth, 
were provided. The pattern has become reason¬ 
ably conventional, and they are entirely satis¬ 
factory in the field, although it is probable that 
a more detailed study would be profitable. A 
selective feature of the amplifiers also tended 
to minimize wind noise, and no trouble was 
encountered from that source. Satisfactory pro¬ 
tection against moisture and moderate rains 
was provided for the 633-A microphone by cov¬ 
ering it with an extremely thin membrane. The 
product sold by the Young Rubber Company 
under the trade name Naturalamb was the most 
satisfactory of those tested. 

Receiving Amplifiers for Single-Frequency 
Measurements. Early measurements were made 
using sound-level meters and cathode-ray oscil¬ 
loscopes. The selective amplifiers used later 
were designed particularly to reduce the effect 
of the complex wave form from the siren source 
but were found very helpful generally in re¬ 
ducing external noise from wind, airplanes, 
motor vehicles, etc. A resistance-capacitance 


network of the type known as Bridged-T was 
used in the negative-feedback path of a high- 
gain amplification stage to reduce the gain for 
all frequencies except one. The theory of the 
circuit has been discussed by Scott 34 and that 
of the Bridged-T network by its inventor, H. W. 
Augustadt, 35 and by Tuttle. 36 The network has 
been shown to operate in a manner similar to 
the familiar Wien bridge. 

Phase Measuring Systems. Several methods 
of phase measurement were used. 

1. Lissajous Figures. Perhaps the best known 
of all phase measuring systems is the use of 
the ellipse pattern produced by placing the sine 
wave signals directly on the horizontal and 
vertical amplifiers of the oscilloscope. While 
accurate quantitative measurement over a con¬ 
siderable range of phase difference is difficult, 
the method is valuable as an accurate way of 
adjusting the amplifiers for equal phase on a 
test signal, for comparing the frequencies of 
the two oscillators, and for rapid qualitative 
observation of phenomena. The earlier studies 
were made with this method with the addition 
of a calibrated, manually operated phase shifter. 
The phase was adjusted by hand to zero, as 
shown on the oscilloscope, and the phase shifter 
reading was recorded. Observations made in this 
manner were necessarily slow and could be 
used only when the atmosphere was quiet, or 
at low frequencies and short distances. 

2. Step-Pattern Method. The first method of 
instantaneously recording phase was based on 
the addition of two square waves. The com¬ 
bined wave was placed on the vertical amplifier 
of the oscilloscope, with the conventional linear 
sweep on the horizontal plates. The traces were 
photographed by a motion picture camera. Con¬ 
siderable difficulty was experienced in using 
this method in the field because instability of 
both the sound-source frequency and the gas¬ 
oline-driven power supply unit made synchroniz¬ 
ing the oscilloscope sweep very troublesome. 

3. Broken-Circle Method. In the step-pattern 
method an ambiguity in phase of 180 degrees 
occurred unless considerable extra equipment 
was added. The broken-circle method was de¬ 
veloped to overcome this disadvantage. As 
shown in the block schematic (Figure 29) the 
90-degree phase shifter (the variable phase 


CONFIDENTIAL 



144 


GUN RANGING AND LOCATING SYSTEMS 


control in Selective Amplifier I) was used to 
form a circle on the oscilloscope screen from one 
of the signal inputs. A fixed step pattern was 
formed from the second signal input by the 
phase shift control of Selective Amplifier II. 


graphs obtained by this method are shown in 
Figure 30. 

4. Final Method. The method of phase meas¬ 
urement finally adopted, because it was found 
most satisfactory in application to direct elec- 



phase i-n 


Figure 28. Direct recording of phase and amplitude. Block schematic. 


The step pattern was impressed on the control 
grid, or so-called z axis of the oscilloscope 
cathode-ray tube, blocking out a portion of the 
circle corresponding to the negative signal peak, 
and leaving a bright spot at the positive peak. 



Figure 29. Broken-circle phase measurement. 

The width of this blocked-out portion could be 
controlled by adjusting the phase network in 
Selective Amplifier II. The phase could be read 
directly from the screen by a common protrac¬ 
tor, and the amplitude at Microphone I could 
be read from the radius of the circle. Photo- 


tromechanical recording, was one based on a 
circuit developed by the Sperry Gyroscope Com¬ 
pany which was made available for this work 
through the courtesy of that organization. This 
method employed the familiar Eccles-Jordan 
trigger circuit and comprised diode limiters and 
the phase meter proper, as shown in the block 
schematic (Figure 28). 

In common with several other methods of 
phase measurement, the system chosen began 
by converting the incoming signals into square 
waves of constant amplitude. It was essential to 
maintain the symmetry of the sine wave form, 
keeping the zero intercepts at their original 
points. The advantages of a symmetrical type 
of limiter, such as the biased diode arrange¬ 
ment, were apparent. The basic principle was 
that of shunting the circuit with two diodes in 
opposite directions, so biased that they con¬ 
ducted as soon as the biasing voltage was ex¬ 
ceeded, effectively chopping off the peak of each 
wave. By amplifying the chopped wave and 
chopping again in a similar manner, a wave 
which was virtually square and of constant am¬ 
plitude was obtained. 


CONFIDENTIAL 

















































SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


145 


The trigger circuit of Eccles and Jordan has 
become an important component in many re¬ 
cent schemes. It consists of two vacuum tubes 
so interconnected that the voltage on the plate 
of each controls the grid voltage of the other in 
such a way that plate current flow in one biases 
the other to cutoff. Thus, at all times, one and 
only one of the tubes is conducting. The action 
is shifted from one tube to the other by the 
application of a signal pulse on the grids. In 
applying this circuit to phase measurement, the 
sine signal received was converted to a square 
wave in the manner previously discussed, and 
was passed through a capacitance-resistance 



Figure 30. Sample record for broken-circle 
method. 

series combination which served as a differen¬ 
tiating network. 37 The resulting sharply peaked 
waves were applied to the grids of the two tubes 
comprising the trigger circuit, which was so 
biased that it responded only to positive pulses 
of energy. Transfer of conduction from one tube 
to the other thus occurred for each positive 
impulse, such as those at a and b under Differ¬ 
entiated Signals in Figure 31. If the potential 
difference between the two plates (or the two 
cathodes) of the trigger circuit were placed on 
an oscilloscope screen, the resultant wave form 
would resemble that of Curve 4 in Figure 31, in 
which the length d of one side of the rec¬ 
tangular pulse followed the proportionality 
A<£/360 = d/\ through the entire 360 degrees. 
If the wave were applied directly to an ordinary 
d-c meter, it would read an average value shown 
by the dashed line in the drawing, because of 
the inability of the needle to follow the rapid 
changes. This d-c reading would be directly pro¬ 
portional to the length of the rectangular pulses, 


and hence to the difference of phase between the 
incoming signals. As applied to the recorder, it 
was necessary to include an integrating network 
of resistors and shunt capacitance to replace 
the natural ballistic characteristic of the meter 
movement. 

Recording Equipment. As indicated above, 
the first measurements were manually recorded. 
Development of the system for photographing 
oscilloscope traces permitted more rapid meas¬ 
urements, and enabled work at longer distances 
and higher frequencies. Reduction of the data 
to usable form, however, required much tedious 
work and could not be done until the films had 
been processed. Extension of the project to in¬ 
clude photographing a number of meteorological 
observations simultaneously was contemplated 
but was abandoned in favor of the direct-re¬ 
cording method. 

A thirteen-channel recorder (see Figure 32) 
was designed and built by the Rahm Instrument 
Company in accordance with requirements. 
Each coil is supported by a beryllium-copper, 
spider-shaped spring suspension, and is linked 
through a simple lever system to a pointer 5 in. 
long, made of two lengths of fine aluminum 
tubing with a reinforcing structure of horse¬ 
hair cloth cemented on. The tip is a short loop 
of Nichrome wire, and a heating current of 
about 0.8 amp at 1.25 v is passed through it. 
Recording is on Thermo-Contax paper, which is 
coated with an easily fusible wax surface; the 
surface is effectively lubricated by the melted 
wax, and there is no measurable damping of the 
pen response from the friction of the paper, as 
in most ink recorders. The unit is easily capable 
of traveling over a range from maximum deflec¬ 
tion to zero in less than y 80 second. A peak-to- 
peak deflection of about 2 in. is available, with 
some departure from linearity at the extremes. 
It was generally limited to 1 in. for improved 
linearity. The thirteen units are mounted in two 
rows and are just 2 in. wide so that, allowing 
1 in. to each pen, the 14-in. paper is covered 
efficiently. 

Truck. The recorder, phase and amplitude 
measuring circuits, meteorological instrument 
circuits, the Webster-Rauland power amplifiers 
and oscillators, and associated test and control 
equipment were all mounted on standard panel 


CONFIDENTIAL 





146 


GUN RANGING AND LOCATING SYSTEMS 


A = 90° 


I ORIGINAL WAVES 



C A0 = 270° 



B A0= 180° 




a 


b 



Figure 31. Trigger circuit—phase measurement system. 


CONFIDENTIAL 
























































































































































































SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


147 


racks in a Ford 1.5-ton panel truck. A set of six 
reels with hand cranks was installed in the rear 
of the truck to accommodate the microphone 
cables and field wire leads to the sound source 
and meteorological instruments. 

Power Supplies. The source of primary power 
in the field was a Universal gasoline-driven 
generator, Model 1500-B. This unit, which 
weighed 410 lb, was rated at 1,500 w, 110-120 
v at 60 c, and was mounted on rubber shock¬ 
absorbing mounts in a standard 4x7 ft Army 
trailer. The trailer was large enough to accom¬ 
modate, in addition, the loud-speaker referred 
to above as the 5-ft horn. The Ford truck and 



Figure 32. Rahm recorder. 

trailer thus provided a complete, independent 
system for field measurements. 

Mast and Associated Equipment. Some of the 
measurements were made as a function of the 
height of the microphones above the ground. A 
50-ft molded plywood radio antenna mast was 
secured for this purpose on loan from the U. S. 
Army. It is shown in Figure 33, representing a 
typical setup during studies at Fort Bragg. The 
sound source—in this case the large stereo¬ 
phonic horn mounted on an Army truck—is 
shown 400 yd distant at A. The movable micro¬ 
phone is located in the wind screen at B, guided 
by two tightly stretched wires passing through 
tubes on the sides of the wind screen, and pulled 
by a third wire which runs through pulleys at 
the top and bottom of the mast. The equipment 
shown on the other side of the mast at D and 


the drum G were used in the meteorological 
studies and will be described subsequently. The 
reference microphone in its wind screen is dis¬ 
cernible on the ground at C, and a third micro¬ 
phone, used in other tests, may be observed im¬ 
mediately below the wind direction indicator E. 

Interpretation of the phase records required 
a high degree of accuracy in the position of the 
moving microphone. Even with the guide wires, 
which greatly reduced swaying and swinging of 
the microphone, it was found desirable to sta¬ 
tion an observer with a surveying transit at a 
point about 200 ft from the mast (not shown 
in Figure 33) in a direction perpendicular to 
that of the sound path. 

In some of the tests a second similar mast 
was used at some point between the sound 
source and the receiver. 

Impulse Measurements 

Impulse measurements were made using both 
the Army GR-3-C system and the Dodar system. 
In some cases modifications of the system were 
introduced. The systems are outlined in Figures 
34 and 35. 

Sound Sources. As sources of impulse sounds, 
only those methods were considered which 
would duplicate, or at least approach, the acous¬ 
tic wave emitted by gunfire or an exploding 
shell. Guns and charges of TNT were used 
mainly for this part of the investigation. Most 
of the long-base work involving the GR-3-C 
recorder was done with guns of various calibers, 
as was also a good portion of the Dodar tests. 
Records were made on both muzzle waves and 
shell-burst waves. For extremely short ranges 
(100-200 yd) an acetylene gun was used. 

Microphones. The only microphones used in 
the measurements on impulse sounds were the 
standard U. S. Army T-21-B sound-ranging mi¬ 
crophones and the T-21-B modified micro¬ 
phones. 

Recording Equipment. The GR-3-C oscillo¬ 
graph and the Dodar have already been de¬ 
scribed. As a means of studying the amplitude¬ 
time difference relationship with Dodar, a 
device was developed for measuring the ampli¬ 
tude of the sound wave simultaneously with the 
measurement of its arrival time by Dodar. The 
amplitude of the first pressure peak was chosen 


CONFIDENTIAL 










148 


GUN RANGING AND LOCATING SYSTEMS 


as the first quantity to be measured, since there 
was no evidence to indicate that any other point 
on the wave was more important to the desired 
correlation. The peak-reading vacuum tube volt¬ 
meter which was developed has two channels 
for reading each of two Dodar inputs. For in¬ 
dicating purposes, it uses two meters (0-1 ma), 
which, with calibration charts, permit determi¬ 
nation of the peak values in terms of db above 
or below 2.45 peak volts. With a combination 


in the transmitted sound. The additional chan¬ 
nels of the recording oscillograph were provided 
to place these factors on the record also as func¬ 
tions of time, and in such a position that their 
relationships to the received sound amplitude 
and phase might be observed. 

The linear dimensions of any practical me¬ 
teorological instrument are considerably smaller 
than those of the wave front distortions called 
corrugations or serrations and the difference 



Figure 33. Typical field setup. 


of the peak-reading voltmeter just described 
and the Dodar, it was possible to make a series 
of measurements at intervals as short as 1/2 
minute. 

Meteorological Measurements 

An important part of the work was the study 
of instantaneous micrometeorological conditions 
at the time of sound recording. Instruments 
were developed to record wind speed, wind di¬ 
rection, and temperature with a response time 
comparable with the fluctuations encountered 


between macro- and micrometeorological equip¬ 
ment resolves itself, therefore, into the time 
rate of response of the device. Macrometeoro- 
logical instruments, being intended for meas¬ 
urements of long-period phenomena, are so 
designed that they average out the rapid fluc¬ 
tuations which are more properly in the domain 
of micrometeorological measurements. In me¬ 
chanical instruments this is accomplished by 
large mechanical inertia, and in thermal instru¬ 
ments by long constants. 

The short time constants required of micro- 


CONFIDENTIAL 












SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


149 


meteorological instruments implies small 
masses for the mechanical instruments and low 
heat capacities for thermal ones; therefore, it 
was expected that the size of these units would 
be small and not very rugged. On the other 
hand, the macrometeorological instruments 
with large time constants may easily be built 
with very rugged construction. 

Mounts for the meteorological units must sat¬ 
isfy the primary purposes of supporting and 


MICROPHONE AND AMPLIFIER 


zO 3 


CONTROL 

OUTPOST 


GUN OR 
EXPLOSIVE 
CHARGE 


D T-2I-B-M' 




D Tt- 21-B-M h 


D | T-2I-B-M 



GR-3-C 

SOUND¬ 

RANGING 

OSCILLOGRAPH 


UP TO 8 CHANNELS-- 1 


Figure 34. Block schematic of oscillographic 

method—impulse sounds. 

protecting the elements, but must not interfere 
with the ability of the instruments to measure 
the characteristic desired in free air. Thus, 
anemometer mounts must be so constructed 
that they will not affect the action of the wind 
on the sensitive element. Thermometer mounts 
must not increase the heat capacity of the unit 
to the point where its thermal lag is too great 
to make it usable, and must shield the unit from 
any direct or indirect radiation without inter¬ 
fering with the free contact of element and air. 

The following sections, which describe the 
instruments actually used, have been separated 
into those dealing with temperature measure¬ 
ments and those dealing with wind measure¬ 
ments. It will be noted that the thermocouples 
were used for temperature measurements only, 
whereas the wire-resistance and thermistor 
units were used for both temperature and wind 
instruments. The difference between the use of 
a resistance-varying element as a thermometer 
and as an anemometer is merely one of the 
temperature to which it is heated by the current 
being passed through it, the former using ex¬ 
tremely low values of current and the latter 
fairly high values. 

Measurement of Temperature and its Fluc¬ 


tuation with Time. Three thermal methods were 
developed and tested for the measurement of 
temperature: 

1. Wire resistance units made of 0.006-in. 
platinum wire. Their use was finally abandoned 
because of their short time constant, which in¬ 
troduced unnecessary complications into the 
temperature record. 

2. Thermistor units, using Western Electric 
thermistor units. 

3. Thermocouples, employing copper and 
constantan. 

Auxiliary equipment was necessary to shield 
these various instruments from direct radiation 
from the sun and also from any reradiation 
by surrounding objects. At night the shields 
were used to prevent radiation to a clear open 
sky. The approach to the problem consisted of 
measuring temperatures at several positions 
beneath the various types of shades under con¬ 
ditions of bright sunlight and light wind. Each 
type of shade developed was chosen in an at¬ 
tempt to improve the previous model. An analy¬ 
sis of the results revealed the following: (1) 
metallic aluminum shading units provided more 
effective shielding than the corresponding ones 
of Bristol board coated with aluminum paint; 
(2) conical shades, with the apex of the cone 
pointing toward the ground, gave more shield- 


XT 

GUN OR 
EXPLOSIVE 
CHARGE 


MICROPHONE AND 
AMPLIFIER 



Figure 35. Block schematic of Dodar system— 
impulse sounds. 


ing than with the apex pointing upward; (3) 
the smaller conical units performed equally as 
well as the larger ones; (4) disk units above 
and below the element gave the maximum 
shielding. 

Measurement of Wind Speed and Direction. 
The problems involved in wind measurements 
are somewhat more complicated because wind 
velocity is a vector in three-dimensional space, 
whereas temperature is a scalar quantity. This 
required the use of several channels on the 
direct-writing recorder in order to determine 


CONFIDENTIAL 


























150 


GUN RANGING AND LOCATING SYSTEMS 


wind velocity completely. Two methods were 
considered: (1) to record the three components 
of velocity on three separate channels, and (2) 
to record the magnitude of wind speed without 
regard to direction on one channel, together 
with one or two additional channels to indicate 
direction. In much of the work it was assumed 
that the vertical component could be disre¬ 
garded. Limitations of equipment and available 



Figure 36. Calibration curve for temperature- 
compensated thermistor anemometer. 


channels constituted the main reasons for ig¬ 
noring the vertical component. 

Instruments used to measure wind fall into 
two major classes: (1) mechanical and electro¬ 
mechanical systems and (2) devices in which 
an electrical parameter, such as the resistance 
of a heated wire or similar element, is made to 
vary directly with the wind. 

Five types of instrumentation were developed 
for the measurement of wind velocities, and 
some work was expended on the development 
of calibrating systems. 

1. Wire resistance anemometers of fine plati¬ 
num wire, heated from 300 C to 1,000 C above 
ambient still-air temperature. This type of in¬ 


strument was used in a number of field tests, 
but it presented some difficulties. It is possible 
that with a careful study of different types of 
hot wires, operating temperatures, and varying 
frequency characteristics in the amplifier, a sat¬ 
isfactory instrument to measure very rapid 
fluctuations might be developed. 

2 . Thermistor anemometers. These units were 
similar to those used to measure temperature, 
but were operated heated. Fairly successful at¬ 
tempts were made to compensate for ambient 
temperature changes and to obtain a response 
linearly proportional to wind speed. Figure 36 
shows the degree of correction achieved with 
a circuit using for R c a Keystone Carbon Com¬ 
pany type LE NTC resistor, which possesses a 
negative temperature coefficient. 

3. Cup anemometers. Conventional rotating- 
cup anemometers were used in some of the 
studies. They were found sensitive to wind 
speeds of 2 or 3 mph, but could not be relied on 
for short-period gusts or changes in the wind. 
Consideration was given to designing a mechan¬ 
ical anemometer of a more miniature type, but 
the problem of introducing critical damping 
without losing quickness of response remained 
unsolved. 

4. Wind direction indicators. A conventional 
wind vane carrying a pointer and calibrated 
scale was used in all the early studies. A very 
simple device for application to direct recording 
was made, using a vane of aluminum sheet, 9x12 
in., mounted by means of a counterweighted 
dowel rod on a ball-bearing-supported rotor, 
which in turn rotated a brush on a potentiom¬ 
eter. Ordinary 0.5-watt, 20,000-ohm composi¬ 
tion resistors were connected between the seg¬ 
ments to form the potentiometer. With a 22.5-v 
battery connected across the terminals, the out¬ 
put was more than enough to produce full-scale 
deflection on the Rahm recorder through the 
regular driver amplifier. The resulting plot in 
the recorder increased steadily in 12-degree 
steps with the angle of rotation. This vane was 
used in many of the field tests and may be seen 
at E in Figure 33. This wind vane was suffi¬ 
ciently sensitive to respond to a steady breeze 
of 1 or 2 mph, but was relatively slow in re¬ 
sponse and would not follow short-period gusts 
of air. 


CONFIDENTIAL 






SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


151 


A system using selsyn motors and a torque 
amplifier was developed which presented very 
little inertia and would respond to small cur¬ 
rents and rapid changes of the order of 0.5 
second with very little tendency to oscillate or 


of certain relationships, it may be more con¬ 
venient to record directly the magnitude of each 
component of wind velocity, rather than the 
speed and direction of the wind. Several devices 
were built, and others considered, to measure 



Figure 37. Phase difference, acoustic amplitude, and wind curves at distances of 200 and 800 yd. 


overshoot. The basis of the instrument was a 
small selsyn unit designed for airplane-meter¬ 
ing applications and sold as Telegon, Type 315- 
F-9013. Some mechanical difficulties were 
encountered in developing a satisfactory 360-de¬ 
gree continuously rotatable potentiometer with 



Figure 38. Dependence of amplitude fluctuations 
on frequency. 


low contact resistance and low frictional losses, 
and the project was not satisfactorily com¬ 
pleted. An adequate answer might lie in the 
commutator type of potentiometer described 
in the direct systems. 

5. Velocity component meters. For the study 


these components by measuring the drag forces 
exerted on a plate, sphere, or other solid, but 
a completely satisfactory instrument was not 
built in the time available. 

Calibrating Systems. Calibration of anemo- 
metric instruments requires either a stream of 
air of known speed, or a means of moving the 
instrument with a known velocity through rela¬ 
tively quiet air. Systems of the first type, ex¬ 
emplified by a small wind tunnel, and of the 
second type, exemplified by torsion pendulum 
and rotating arm systems, were employed. 

General Comments. The development of mi- 
crometeorological instruments deserves further 
attention. It was felt by the Division that its 
work in that direction resulted in some contri¬ 
butions and suggestions which should prove of 
value in the future. One further suggestion 
which should be mentioned is the desirability of 
developing means of measuring quantitatively 
the acoustic parameters of a given terrain; it 
has only recently become apparent that a study 
of terrain (though not generally regarded as a 
meteorological factor) is highly important. 


CONFIDENTIAL 































































152 


GUN RANGING AND LOCATING SYSTEMS 


Experimental Results 

Effectiveness of the Standard Meteorological 
Correction. In determining the azimuth of a 
sound source by means of a pair of microphones, 
application of the standard meteorological cor¬ 
rection statistically improves the results. Since 
the effectiveness of the standard method varied 
considerably at different times, the following 



SOUND FREQUENCY 


MICROPHONE HEIGHT = 45 IN. MICROPHONE SEPARATION = 40 FT 
DISTANCE FROM SOURCE = 300 FT WIND VELOCITY= 6 MPH 

Figure 39. Acoustic frequency dependence on 
phase fluctuations. 

two suggestions are made. (1) Whenever pos¬ 
sible, use the average of several locations of a 
sound source, rather than a single determina¬ 
tion. (2) When meteorological conditions are 
changing, obtain meteorological messages fre¬ 
quently, rather than at regular prescribed in¬ 
tervals, usually of several hours’ length. 

The determination of the effective wind is 
open to the serious objection that the averaging 
of the weighted winds does not treat them cor¬ 
rectly as vectors. In addition, the whole method 
of weighting is empirical and can be expected to 
be satisfactory only on the average. Therefore, 
it is suggested that the effective wind should be 
determined by plotting the weighted minute 
winds as vector quantities in a manner similar 
to the plotting of the ballistic wind for artillery 
purposes. 

The determination of the effective tempera¬ 
ture is subject to criticism on several counts. 
One is that the choice of the temperature at 
500 ft seems rather arbitrary. Another diffi- 




D IN YARDS 

Figure 40. (Top) Variation of phase deviation 
with distance. (Bottom) Variation of per cent 
amplitude deviation with distance. 

culty is the assumption that the temperature at 
500 ft is 2 F less than that at the ground. This 
certainly is not the case when there is a tem¬ 
perature inversion. 


CONFIDENTIAL 






































SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


153 


Early Experiments on Micrometeorological 
Fluctuations. In the earlier experiments em¬ 
ploying one or more pairs of microphones, the 
two microphones of a pair being equidistant 
from the source, and a steady sound source of a 




0 20 40 60 80 100 120 140 160 180 


MICROPHONE SEPARATION IN FEET 

Figure 41. Phase deviation as a function of 

microphone separation for perpendicular arrays. 

given frequency, the following general charac¬ 
teristics were usually found: 

1. Rather large fluctuations in phase differ¬ 
ence, of an order corresponding to fluctuations 
of 1 millisecond with a microphone separation 
of 40 ft; 

2. Rather rapid fluctuations in phase differ¬ 
ence, a complete cycle of this fluctuation fre¬ 
quently occurring in a 20-second period; 

3. A mean phase difference not equal to zero; 

4. Rapid, large changes in wind speed and 
direction; 

5 . No obvious correlation between instan¬ 


taneous values of phase difference and wind 
speed and direction. 

Magnitude of Micrometeor ological Fluctua¬ 
tions. The difference in the times of arrival of 
a sound impulse from a given source may vary 
rapidly with time. Guns, mortars, shell bursts, 
and TNT were used as sources and the GR-3-C 
and Dodar as recorders and indicators. The 
standard deviation of the time interval errors 
for a group of shots showed values ranging 
from 1.5 milliseconds to more than 30 milli¬ 
seconds, the smaller values being obtained at 
the shorter ranges and smaller microphone sep¬ 
arations. As a result of this it is concluded 
that deviations in the difference of arrival times 



Figure 42. Standard deviation in phase difference 
as a function of per cent standard deviation in 
amplitude. 


due to rapidly changing conditions may be a 
serious source of error in sound ranging and 
that further effort to reduce this source of error 
is justified. 

Relationships for Steady, Single-Frequency 


CONFIDENTIAL 


































154 


GUN RANGING AND LOCATING SYSTEMS 


Sources. In an effort to investigate further the 
short-period fluctuations referred to above, ex¬ 
periments with a steady sound source of a given 
frequency were performed, and the relation¬ 
ships between the amplitude of the wave and 
the time of arrival under varying conditions 
were studied (see Figure 37, 10a obtained by 
reading amplitudes and phase difference for two 
microphones every % second). It was found 
that the magnitude of the amplitude fluctua¬ 
tions at one microphone is roughly proportional 
to the frequency of the sound (Figure 38) and 
that the magnitude of the fluctuations in the 
phase difference for two microphones increases 
approximately linearly with the frequency 
(Figure 39). It was concluded, therefore, that 
the mean time deviation is independent of the 
frequency of the sound. 

The magnitudes of the fluctuations in both 
amplitude and phase difference increase with 
the distance from the source. For the phase 
difference, the relationship is roughly linear, 
with possibly a leveling-off tendency at large 
distances as obtained from data in Figures 37 
and 40. 

The magnitude of the fluctuations in phase 
difference increases with the microphone sep¬ 
aration, but this increase is not linear (except 
when the microphones and source are in line), 
and there appears to be a leveling-off tendency 
at the larger separations (Figure 41). Since 
the mean difference in arrival times for a pair 
of microphones increases linearly with their 
separation, it follows that (1) the mean azimuth 
error due to this effect decreases with increas¬ 
ing microphone separation, and (2) short-period 
meteorological fluctuations are a more serious 
source of error in sound ranging when the mi¬ 
crophone separation is small, as for the Dodar 
sub-base, than when the separation is large, 
as in the case of a 2- to 4-sound-second sub¬ 
base. 

The magnitudes of the fluctuations in both 
amplitude and phase difference are greater in 
the daytime than at night. 

The magnitudes of the amplitude and phase 
fluctuations are greatest on gusty days, or days 
of atmospheric turbulence, but no quantitative 
data sufficient to establish exact relationships 
were obtained. 


When the magnitude of the fluctuations in 
amplitude is great, the magnitude of the fluc¬ 
tuations in phase is also great. More specifically, 
for small amplitude variations the time differ¬ 
ence variation is approximately proportional to 
the per cent amplitude variation, but for 
greater fluctuations the time difference varia¬ 
tion is a rapidly increasing function of the per 
cent amplitude variation (Figure 42). Conse¬ 
quently, the seriousness of errors in sound 
ranging due to short-period meteorological 
fluctuations increases rapidly with the degree 
of turbulence, and current sound-ranging meth¬ 
ods become ineffective when turbulence is ex¬ 
cessive. 

The magnitude of the fluctuations in phase 
difference is large when the amplitude of the 
received sound is low, and vice versa. This low 
sound intensity emphasizes the difficulties of 
sound ranging in a highly turbulent atmos¬ 
phere. 

It is highly probable that there is a correla¬ 
tion between the variations of the amplitudes of 
the sound received at two microphones and the 
variations of the phase difference between the 
two microphones (see Table 4 for typical data). 
This correlation may be stated as follows: that 
portion of a wave front arriving earlier than 
the average usually has an intensity below the 
average, whereas a portion arriving later than 
the average usually has an intensity above the 
average; that is, the correlation coefficient be¬ 
tween phase and amplitude is usually negative. 
Although the long-time correlation coefficient 
is usually negative, it was observed that for 
short periods of time it may reverse itself and 
become positive. However, the periods of re¬ 
versal, during which there is no correlation, are 
relatively short and infrequent. Typical results 
are shown in Figure 43, which refers to points 
taken off a continuous record (Rahm recorder) 
at intervals of 1.36 sec; in this particular series 
of readings, a single microphone was placed 
50 ft above the ground at a distance of about 
200 yd from the horn, and the phase of the 
microphone measured relative to that of the 
source. Such a correlation alone would not 
suffice to improve sound ranging greatly, but 
combined with some other factor or measure¬ 
ment which would show the sign of the correla- 


CONFIDENTIAL 



SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


155 


tion, it might result in considerable improve¬ 
ment. 

The possibility of correlating the sign of the 
phase-amplitude correlation coefficient with one 
or more micrometeorological factors should be 
investigated. Much of the necessary equipment 
for such an investigation was developed, but 
time did not permit field tests with it. 

Should such an investigation prove successful 
in correlating the sign of the phase-amplitude 
correlation coefficient with measurable micro- 
meteorological factors, the possibility will exist 


poor, and the errors due to the micromet are 
large. 

Relationships for Impulse Sources. Similar 
measurements were made using sources of 
sound impulses and both long and short bases. 
The results obtained were completely consistent 
with those obtained with single-frequency 
sources. Two examples follow. 

1. For impulse sounds the standard deviation 
of the error in the arrival time interval for two 
microphones increases with the distance of the 
source. 


Table 4. Phase difference vs amplitude. 


Number and 

Separation of 
microphones 

Number of 
readings 

Number of 

Correlation 

Probability of 
distribution 

time of record 

(ft) 

in group 

groups 

coefficient 

being random 


8 

20 

80 

8 

—0.519 

0.2 

1605 


40 

16 

—0.545 

0.03 



20 

33 

—0.358 

0.05 



10 

66 

—0.308 

0.01 

9 

80 

80 

9 

—0.563 

0.1 

1759 


40 

18 

—0.674 

0.002 



20 

36 

—0.448 

0.005 



10 

72 

—0.327 

0.005 

10 

20 

80 

8 

—0.289 

0.5 

1812 


40 

16 

—0.309 

0.2 



20 

33 

—0.447 

0.01 



10 

66 

—0.491 

<0.001 

10 

160 

80 

8 

—0.760 

0.03 

1812 


40 

16 

—0.513 

0.04 



20 

33 

—0.557 

0.001 



10 

66 

—0.506 

<0.001 

20 

80 

80 

10 

+0.261 

0.5 

0410 


40 

20 

+0.268 

0.3 



20 

40 

—0.057 

0.7 



10 

80 

—0.130 

0.3 

23 

160 

80 

10 

—0.036 

0.9 

0755 


40 

20 

—0.175 

0.5 



20 

40 

—0.144 

0.3 


of applying the phase-amplitude correlation to 
correct those time difference errors in sound 
ranging which are due to short-period meteoro¬ 
logical fluctuations. 

There is some indication that the phase- 
amplitude correlation is good when no tempera¬ 
ture inversion exists, wind speeds are high, and 
the air turbulent, and that the correlation is 
poor when a temperature inversion exists, wind 
speeds are low, and there is little turbulence. 
Such an indication suggests the possibility of 
applying the correlation when it is most needed, 
namely, when sound-ranging conditions are 


2 . For impulse sounds the error in the arrival 
time interval for two microphones and the am¬ 
plitude ratio in decibels for the same pair of 
microphones show a considerable degree of cor¬ 
relation. The sign of this correlation is usually, 
but not always, negative. (See Figure 44, 10b 
which is typical of the results obtained.) There 
is also some indication that this correlation is 
greatest when meteorological conditions are not 
favorable for sound ranging. 

Vertical Structure for Steady, Single-Fre¬ 
quency Sources. Experiments were made using 
single-frequency sources and measuring ampli- 


CONFIDENTIAL 







156 


GUN RANGING AND LOCATING SYSTEMS 


tude and phase as a function of the height above 
the ground. Typical results are shown in Figures 
45 and 46. 

Above dry, plowed ground a minimum in am¬ 
plitude of the sound occurs, not at the ground, 
but at a height which is roughly proportional 
to the wavelength (between 5 and 10 ft for 


perature gradient as well as the frequency. 
Below 400 c there is no pronounced amplitude 
minimum above the ground (Figure 46). 

Above very wet, but not water-soaked, ground 
both the meteorology and the terrain are con¬ 
trolling factors. 

The above results may be summarized as fol- 



200 c), whereas the phase, corrected for varying 
distance to source, increases with height (see 
Figures 45 and 47). These amplitude and phase 
characteristics are independent of the existing 
temperature gradient. 

Above water-soaked ground the amplitude 
and phase structure is dependent on the tern- 


lows: the vertical amplitude and phase struc¬ 
ture above water-soaked ground (acoustic im¬ 
pedance high) is determined predominantly by 
the temperature and wind structure, whereas 
above dry ground (acoustic impedance low) the 
terrain is the predominant factor. In intermedi¬ 
ate cases, when the ground is wet but not water- 


CONFIDENTIAL 































SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


157 


soaked, both meteorological factors and terrain 
affect the amplitude and phase structure. 

The importance of terrain as a factor affect¬ 
ing sound propagation increases as the acoustic 
impedance of the ground decreases. (Acoustic 
impedance of the ground, in general, is de¬ 
creased by increased porosity, as when the 
ground dries out, and by growth of grass or 
other vegetation.) 

There are indications that under certain con- 


types of further investigation might be de¬ 
sirable. 

Persistence of a Distortion of a Wave Front. 
The discussion pf this problem was limited to 
the study of the persistence of a spherically 
shaped boss or bump on an otherwise plane 
wave front in a stationary, homogeneous 
medium. 5 

The specific problem considered is shown in 
Figure 48. The wave front at time t is denoted 


o 

Ll) 

if) 

2 


< 

I 


30 


20 


10 


-20 


-30 


-40 






x 





o - 

INTERVAL (2-1 

I 

c 

CL 





A 



A - INTERVAL (3-2) 

□ - INTERVAL (4-3) 

Y _ iliTrnui . /, 



"T-x- 

X 







initn v« 

INTERV/5 

L. \ O -« 

tL (6 -; 

— 

5) 


A 

—o— 


Tx 3 ' 

x 

Q 

O 



AMPLI 

ITUDE R 

ATIO IN 

DB 



A 

□ > 

.. o. 

A. 

X^~"" 


LJ- 

A 

LEA 

1 

ST SOU, 

r~ 

ARES L 

r 

INE 




A 

□ 

£ 

x 

A. 

5^ 


□ 

o 

30^- 


—A — 






>- 






— A— 

X 



i n i tAruuoiyns 

FT BRAGG 5-20 

_1_1_l 

i-43 











LEAST SOUARES EOUATION! y = -2.76 X + 0.35 
CORRELATION COEFFICIENT * - 0.522 

Figure 44. Time error vs corrected amplitude ratio. 


ditions sound ranging may be improved consid¬ 
erably by elevating the microphones well above 
the ground. 


Theoretical Work 

The preceding sections have dealt mainly 
with the description of apparatus and methods 
of measurement, and with the analysis of data 
largely from a statistical point of view. In the 
remaining sections there will be discussed vari¬ 
ous theoretical problems which served to guide 
the experimental work and to indicate what 


by its plane section FPF, with the spherical 
bump having its center at P. The radius of the 
sphere is R, and the radius of the circular re¬ 
gion in the wave front occupied by the bump 
is u. The advance of the wave front at P is 
measured by v. In the computations carried out, 
R was taken as 20 ft, u as 17.32 ft, and v as 
10 ft. 

The method used for solving the problem was 
that developed by Helmholtz in his analytical 
formulation of Huygen’s principle. The velocity 
potential was computed at a point A on the 
axis OP (extended) and at a point B on the 
perpendicular through the axis where AB = u. 


CONFIDENTIAL 










































158 


GUN RANGING AND LOCATING SYSTEMS 


This was done for different distances x along 
the axis and for different wavelengths A of the 
harmonic sound radiation assumed. For the “on- 
axis” case (point A) the contributions to the 
velocity potential due to the plane and spherical 
wave fronts were computed separately. The 
fundamental idea used in the “off-axis” case 
(point B ) was to add the contributions at B 
due to the spherical wave and due to a plane 
wave over the whole wave front, and then to 


The connection between this and the general 
problem of sound ranging lies in the fact that 
sufficiently persistent irregularities in the wave 
front would result in a change in the times of 
arrival of the wave front at the different micro¬ 
phones of an array, and thereby require for 
their correction information over the whole 
sound path and hence into enemy territory. 
However, if the irregularities are random, 
short-time effects, they may be correctable by 


DATE! 8-28-44 
5 - FOOT HORN 
200 C 


time: 1449 
distance: 200 yd 

O--O UP 


MASON FARM 
GROUND DRY, PLOWED 
A—A DOWN 


1441-1450 

OVERCAST 






DATE: 8- 28-44 
5-FOOT HORN 
200 C 


TIME’. 1500 

distance: 200 yd 

O-O UP 


MASON FARM 
GROUND DRY, PLOWED 

A—A down 


1513- 1523 
OVERCAST 
SLIGHT RAIN 


Figure 45. Amplitude and phase vs height over dry, plowed ground. 


subtract from that sum the contributions at B 
due to a plane wave over an area equal to that 
of the circle cut out by the spherical wave at 
the original wave front. 

The calculations showed that the persistence 
distance depends on the wavelength. If the ar¬ 
bitrary assumption is made that the bump will 
be considered to be smoothed out when the dif¬ 
ference in times of arrival of the wave front at 
A and B is less than 0.02 millisecond, then it is 
found that for a wavelength of 10 ft, the per¬ 
sistence distance is 300 ft; for a wavelength of 
50 ft, about 1,000 ft; and for a wavelength of 
100 ft, about 500 ft. 


using data obtained in the vicinity of the micro¬ 
phone base line, that is, within our own lines. 
Therefore, it was considered desirable to con¬ 
centrate on an experimental study of conditions 
in the immediate vicinity of the microphones. 

The Acoustic Transmission Properties of a 
Moving Thermal Lamina of Air. Consider a 
plane wave incident on a sheet or lamina of 
moving air. Suppose the thickness of the sheet 
is l (see Figure 49), the temperature of the 
sheet is T x = T + AT, where T is the temper¬ 
ature of the surrounding medium in degrees 
absolute, the static pressure in the sheet is the 
same as that of the surrounding medium; and, 


CONFIDENTIAL 



















































































SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


159 


further, suppose the sheet’s velocity is V v paral¬ 
lel to the boundary. 

The problem was to determine the acoustic 
transmission characteristics of this sheet, both 
as regards the amplitude and the phase of the 


DATE: 10-17-44 


FLETCHER HORN 200 C 


time: 1616 

DISTANCE: 215 YD 

A-A up 


MASON FARM 
GROUND WATER-SOAKED 
O--O DOWN 





time: 1628 

DISTANCE*. 215 YD 

A-A up 



120 140 160 180 200 220 

PHASE 

MASON FARM 
GROUND WATER-SOAKEf 
O-o DOWN 


DATE*. 10-17-44 FLETCHER HORN 200 C 

Figure 46. Amplitude and phase vs height over 
water-soaked ground. 


transmitted wave. This was done for sheets of 
arbitrary thickness. The discussion, which is 
largely qualitative, is based on fundamental 
hydrodynamics. 

This study was undertaken to investigate a 
possible mechanism to explain the observed 
phase-amplitude correlations. A plane wave 
will traverse a warm sheet in a shorter time 
than if the sheet were cold, and it will traverse 
a sheet which has a component of velocity in 
the direction of acoustic propagation in a 
shorter time than one whose component of 


motion is opposite to the direction of acoustic 
propagation; in every cas'e, however, the trans¬ 
mission coefficient will be less than 1, since 
reflection will always occur at each of the 
boundaries. Hence, if a sound source is equi¬ 
distant from two microphones, the microphone 
leading with respect to phase will have the 
smaller amplitude when warm pockets of air 
predominate, and the larger amplitude when 
more cold pockets are present. The following 
section gives a quantitative treatment of this 
phenomenon. 

Multiple Lamina. When a sound wave passes 
through a number of sheets of the type just 
discussed, the resulting amplitude and phase 
will obviously depend on their relative orienta¬ 
tions, spacing, temperatures, and velocities. It 
is only for the simplest conditions that a prac- 


i 



O 4 0 12 

WAVELENGTH IN FEET 


Figure 47. Height of minimum in amplitude at 
various acoustic wavelengths (dry, plowed 
ground). 


tical solution is possible. The sheets were as¬ 
sumed to be parallel, equally spaced, and 
identical. Furthermore, it was assumed that 
the reflected energy from each sheet is very 
small, and once reflected may be neglected. 


CONFIDENTIAL 























































160 


GUN RANGING AND LOCATING SYSTEMS 


Three cases were investigated. First, it was 
supposed that the difference in phase and am¬ 
plitude at the two microphones is due to a 
different number of sheets over each path, the 
sheets being identical and having the same in¬ 
clinations with respect to their associated wave 
normals. Secondly, the case was considered 



Figure 48. Persistence of wave front corruga¬ 
tions. 


where the number of sheets over each path is 
the same, the difference in the two paths being 
in the angle of inclination of the sheets with 
respect to the wave normals. Finally, attention 
was given to the relationships obtained when 
there is a difference in the thermal or mo¬ 
mentum content of the sheets. 

The results of the theory did not appear to 
be of the correct order of magnitude to account 
for the experimental observations on phase- 
amplitude correlations; only for angles near 
grazing incidence could anything like quanti¬ 
tative agreement be found. 

Acoustic Phase and Amplitude Relations in 
a Stationary Medium with a Linear Vertical 
Sound Velocity Gradient. A stationary atmos¬ 
phere is generally described as being stratified 
vertically with respect to the temperature (and 
consequently sound velocity). The temperature 
changes are generally gradual, so that no sharp 


boundaries exist. The most successful treat¬ 
ment of the propagation of sound through such 
a medium is one which describes the ray paths; 
the problem here discussed was treated in this 
fashion. The problem was one of determining 
the time of travel and the amplitude of a sound 
ray when the sound velocity gradient is linear 
and positive, account being taken of attenuation 
and spreading of the waves from a point source. 
The ray path considered was one between a 
source and a receiver, both placed on the 
ground. Rays reflected from the ground were 
disregarded. 

A stationary atmosphere was considered. It 
was assumed to have horizontal homogeneity 
and a vertical temperature gradient obeying 
the law 


t = r„(i + »J’, (6) 

where T is in degrees absolute, and the x and y 
axes have been chosen so that the x axis is 
horizontal. Then if c is the velocity of sound 

c = Co ( 1 + l)- (7) 

It is easy to show without mathematical dem¬ 
onstration that the sound rays are circles. 

Analysis showed that as the gradient in¬ 
creases (that is, a decreases) both the ampli¬ 
tude and time of travel decrease. Suppose an 
idealized system of the following type is as¬ 
sumed: (1) two microphones are equidistant 
from a source; (2) a lack of horizontal homo¬ 
geneity in the temperature causes the rays to 
the two microphones to have different values 
of a; (3) the lack of horizontal homogeneity 
is such that it does not alter the circular char¬ 
acter of the rays. It then follows that the micro¬ 
phone at which the amplitude is least is ahead 
in phase (smaller time of travel) with respect 
to the other microphone. 

Of interest is the magnitude of the temper¬ 
ature gradient necessary in order that signifi¬ 
cant time differences be obtained. In order that 
a difference of 1 millisecond be present between 
straight-line and curvilinear propagation over 
a distance of 800 yd, it was found that a must 
be about 8,000 yd. This corresponds to a tem¬ 
perature gradient of about 1 C in 13 yd. Though 


CONFIDENTIAL 












SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


161 


this may be large from a macrometeorological 
standpoint, it seems possible that gradients of 
this order and higher may be present near the 
ground over short periods of time. This is per¬ 
haps to be anticipated, since large amplitude 
changes are to be expected mainly when there 
is a deviation from a linear velocity gradient, 
so that focusing of sound rays occurs. 

Here again the object was to investigate the 
phase-amplitude relationship under specified 
conditions. The results were encouraging, al- 



Figure 49. Idealized transmission through 
thermal lamination of air. 


though rather extreme conditions had to be 
postulated in order to obtain results of the 
order of magnitude of those observed experi¬ 
mentally under uncontrolled conditions. 

Phase-Amplitude Relationships in the Inter¬ 
ference Field of Two Plane Waves. The problem 
was to determine the phase and amplitude pat¬ 
terns when two plane waves whose frequencies 
are the same and whose normals are at an angle 
with each other interact. 

The problem arose from a consideration of 
the interaction between waves being propa¬ 
gated directly and those arriving at a receiver 
after being reflected from the ground. Both the 


magnitude and sign of the phase-amplitude 
relationship in this situalion were shown to be 
dependent on the locations and spacing of the 
receivers, and on the frequency of the sound. 
Thus it was found that the surfaces of equal 
phase do not coincide with the surfaces of equal 
intensity. 

Sound Propagation along a Boundary. The 
propagation of sound at grazing incidence has 
considerable practical importance. In gun rang¬ 
ing the source of sound is always on the ground, 
as is the receiver; a similar situation occurs 
for the great majority of outdoor acoustic 
problems. Thus the results of this study will be 
found to have bearing on such questions as 
altitude listening and velocity and intensity 
investigations. 

The problem of the reflection of a plane wave 
from a boundary separating two semi-infinite 
media has received considerable attention. The 
medium above the plane z = 0 is assumed to be 
uniform; that below the plane has different 
properties and is also uniform. If, then, a plane 
wave in the upper medium impinges on this 
boundary, reflection will ensue, and there will 
also be a refracted wave which penetrates the 
lower medium. It is easily demonstrated that 
the boundary conditions are satisfied when (1) 
the normal particle velocities at the boundary 
in both media are equal at all times, and (2) the 
pressures at the boundary in both media are 
equal at all times—if the refracted and reflected 
waves are plane waves having the same fre¬ 
quency as that of the incident wave. 

The reflection coefficient (for the amplitude) 
takes the form: 

= Z 2 cos fli - Z x cos fl 2 
Z2 COS d\ -f- Z\ COS 62 

where 

Z 2 = complex specific acoustic impedance of 
the lower medium 

Zi = complex specific acoustic impedance of 
the upper medium 

61 = angle of incidence 

62 = angle of refraction. 

The angle of reflection is equal to the angle of 
incidence. 

Practically, plane waves never occur; they 


CONFIDENTIAL 











162 


GUN RANGING AND LOCATING SYSTEMS 


are a mathematical fiction which can be only 
approximated physically. Consequently, the ex¬ 
tent to which equation (8) applies to practical 
problems becomes a matter of experiment. It is 
indeed gratifying that the reflection coefficient 
given by equation (8) holds even for very 
rough approximations to the plane wave. Thus 
it has been found to hold for point sources of 
sound at relatively small distances from reflect¬ 
ing surfaces. This has led to the practice of 
applying the reflection coefficient to rays of 
sound. 

There are limitations to the applicability of 
equation (8) which are immediately apparent 



Figure 50. Propagation along a boundary. 


on closer examination. Regardless of the rela¬ 
tive magnitudes of Z x and Z 2 at grazing inci¬ 
dence (0i = 90 degrees), R = — 1. (The re¬ 
flected wave is equal in magnitude and 180 
degrees out of phase with the incident wave.) 
Thus, for plane waves, the reflected wave com¬ 
pletely cancels the incident wave, and there is 
no energy propagated along the boundary. On 
an experimental basis this is impossible, and 
one is led to the conclusion that the assumptions 
made in deriving equation (8) cannot be real¬ 
ized physically. However, this was recognized 
to be true at all angles of incidence, so one must 
conclude that, especially at grazing incidence 
(and for angles close to grazing), it is impos¬ 
sible adequately to approximate the plane wave 
conditions which were assumed, and indiscrim¬ 


inate application of equation (8) may lead to 
serious error. 

To obtain a solution of the problem at angles 
near grazing incidence, it was necessary to set 
up the problem with a closer correspondence 
to physical reality. A point source of sound was 
assumed instead of a plane incident wave, the 
incident wave then being spherical. The form 
of the reflected wave was determined by the 
boundary conditions. 

The problem here discussed has arisen in 
electromagnetic theory. Sommerfeld 38 attacked 
the problem first in 1909, and there has been 
considerable subsequent work. The analysis is 
rather involved, and it is fortunate that the 
acoustic case may be treated in a closely similar 
fashion. When the electromagnetic source is a 
vertical dipole, the boundary conditions ex¬ 
pressed in terms of the Hertzian vector are 
similar in form to the acoustic boundary con¬ 
ditions expressed in terms of the velocity po¬ 
tential. 

It was assumed that the material is isotropic. 
A porous material is characterized by the fact 
that a large part of its volume is taken up by 
air pores and channels. The solid material itself 
will have a density and rigidity which is high 
compared to that of air. Consequently, when an 
air sound wave impinges on the porous mate¬ 
rial, because the reflection coefficient of the 
solid material is likely to be very high, the wave 
will tend to remain an air wave, and energy 
will be transmitted mainly by the air in the 
pores. 

It is evident that the acoustic properties of a 
porous material will be a function of frequency. 
One may also expect a behavior which is a 
function of the size of the pores or tunnels, 
wavelength being constant. 

Suppose a point source to be a distance z a 
above a plane, below which lies a medium which 
has a propagation constant k 2 and complex 
density P2 (see Figure 50). The problem was to 
find the sound field at a point a horizontal dis¬ 
tance r 0 from the source, and a distance z b 
above the plane. 

The theory developed led to interesting re¬ 
sults concerning loss in intensity along a bound¬ 
ary. The loss is found to be proportional to the 
square of the frequency. One is thus led to 


CONFIDENTIAL 







SOUND TRANSMISSION THROUGH THE ATMOSPHERE 


163 


expect that a complex tone of low frequency 
will lose its harmonics at great distances to a 
much greater degree than air absorption, scat¬ 
tering, etc., would indicate. This is a generally 
observed phenomenon in atmospheric acoustics 
involving low-frequency sound waves. Under 
certain circumstances the theory indicates that 
there may be a consistent 12-db drop every 
time the distance is doubled, even when there is 
no absorption in the air. The 12-db value is the 


50 


40 

H 

Ul 

u 

^ 30 

z 

£ 20 

o 

ui 

X 10 


0 

DB ABOVE MINIMUM AMPLITUDE 




180 160 140 120 100 80 60 40 20 


PHASE IN DEGREES 

Figure 51. Observed and calculated phase- 
amplitude structure over dry ground. 

sum of the effects of divergence from a point 
source and the boundary loss function. 

The theory predicts a phase lag. This phase 
lag would become important in velocity meas¬ 
urements. For example, if a measurement of 
velocity were made using a 2,000-c wave by 
measuring the number of waves between 5 cm 
and 50 cm over Quietone, the source and re¬ 
ceiver being at the boundary of the material, 
the error introduced by this phase lag would 
result in a value which was low by 7 per cent. 


With the source taken at the boundary, the 
amplitude and phase structure for various re¬ 
ceiver heights was calculated for air above 
Quietone and air above earth, the calculations 
in the latter case being based on measured 
values of flow resistance and porosity. The 
following results were obtained: (1) there is 
a minimum amplitude above the boundary; (2) 
the height of this minimum is practically inde¬ 
pendent of distance; (3) the rate of increase 
of the amplitude with height above the mini¬ 
mum is greatest at the greatest distance; (4) 
the height of the minimum increases with in¬ 
crease in wavelength; (5) the phase increases 
with height, the biggest increase occurring at 
the smaller heights; (6) at greater heights, 
agreement with classical theory is approached. 

Figure 51 shows the observed and calculated 
phase-amplitude structure over dry ground. 
Curve 1 was calculated for a flow resistivity 
of 84 and a porosity of 54 per cent, and Curve 2 
for a flow resistivity of 168 and a porosity of 
54 per cent, values approximating those found 
experimentally for soil samples. Curve 3 is an 
experimental curve similar to Figure 45. It is 
seen that the experimental curve is bracketed 
by the calculated curves. 

The rapid increase of the amplitude with 
height above the minimum at great distances 
suggests important applications to such fields 
as altitude listening. 

Sound Transmission through a Rectilinear 
Vortex . This and the following section refer to 
theoretical considerations having a direct bear¬ 
ing on the source of wave front corrugations or 
irregularities. 

The problem of sound transmission through 
a rectilinear vortex was treated, using hydro- 
dynamical principles. It is shown that the ob¬ 
served phase fluctuations may well be due in 
part to the passage of a sound wave through 
simple vortices. In general, to be really effec¬ 
tive in producing a time difference, the vortex 
center must pass across the line joining the 
source to the center of the base line. 

Figure 52 gives a rough picture of what 
would be expected to happen when a sound 
wave originally plane approaches a vortex. The 
rotation is taken as counterclockwise. The 
dotted lines are the traces of the successive 


CONFIDENTIAL 































164 


GUN RANGING AND LOCATING SYSTEMS 


equiphasal planes as they would be without the 
presence of the vortex. The full lines represent 
the actually distorted equiphasal planes. Since 
wave fronts are equiphasal surfaces, it is clear 
that the phase at P" leads that at P'. On the 
other hand, if the amplitude at any point like 
P" is considered to be made up of contributions 
from all parts of the immediately preceding 
wave front, it seems necessary that the inten¬ 
sity at P" shall be less than at P', since the 
disturbances which reach P' will be in phase 
from more parts of the wave front near P' than 
is the case for P". Stated another way, near 
P" the wave disturbance is moving out, more 
or less like a diverging cylindrical wave, 
whereas near P' it is moving in, more or less 
like a converging cylindrical wave. 

It seems reasonable to conclude that acoustic 



Figure 52. Phase diagram of a plane wave pass¬ 
ing through a vortex. 


wave front corrugations can be caused by the 
presence of simple circular vortices in the radi¬ 
ation field. It also appears that the distortion 
due to a vortex is such that an increase in phase 
is associated with a decrease in intensity and 
vice versa. 

Scattering of a Plane Sound Wave by a Sta¬ 
tionary Cylindrical Fluid Obstacle. A possible 
cause of variations in the wave front of a sound 
wave is the presence of regions of higher or 
lower temperature in the medium between the 
source and the receiver. These scatter the sound 


and hence can produce phase and intensity 
fluctuations at any pair of receivers. 

A theoretical study was made of the phase 
and amplitude relationships resulting when a 
plane sound wave passes through a cylinder of 
air, at a temperature higher than that of its 
surroundings. Both the effect of distance on 
the magnitude of the phase and amplitude 
changes caused by the cylinder, and the rela¬ 
tionship between the phase changes and the 
amplitude changes were considered. 

In general the theoretical results confirm the 
hypotheses that the effect of a variation in the 
wave front tends to spread and later become 
inappreciable, and that an advance in phase is 
associated with a decrease in amplitude, and 
vice versa. 

Hypothesis of Regional Effectiveness. An hy¬ 
pothesis was introduced stating that the wind 
is effective in influencing a microphone only 
while it is in a certain region about that micro¬ 
phone. Computations using some of the ob¬ 
served data yielded 56 ft as the region of ef¬ 
fectiveness. (Only the order of magnitude is to 
be considered as significant.) Using some sim¬ 
plifying assumptions, a relation was derived 
which predicts that the mean phase deviation 
will increase linearly with the microphone sep¬ 
aration for small separations, and will then 
increase much more slowly as the separation 
increases. This result agrees with the observed 
data. In its present form the hypothesis fails to 
afford a means of predicting theoretically the 
size of the effective region. 

Conclusion. It is unlikely that a single influ¬ 
ence is continuously predominant in causing 
acoustic wave front corrugations except under 
unusual meteorological conditions. However, 
regions of varying temperature and air circu¬ 
lation are probably frequent causes of wave 
front distortions. It seems quite possible that 
a theoretical basis for a phase-amplitude cor¬ 
relation exists. 

5 6 ACCESSORY PROJECTS 

5 61 Binaural Listening 13 

The object of this work was the development 
of a binaural listening device to be used as a 


CONFIDENTIAL 







































ACCESSORY PROJECTS 


165 


sound-ranging outpost and as an anti-infiltra¬ 
tion device for the detection of enemy move¬ 
ments in the vicinity of patrols or important 
stations. The development of the binaural out¬ 
post (Binop) has already been described (Sec¬ 
tion 5.3). Discussion here will be limited to the 
anti-infiltration sets. 

Advantages of an Anti-Infiltration Listening 
System. An anti-infiltration listening system is 
a device by which enemy movements may be 
detected over a wide range of territory by one 
observer without the need for personnel at the 
sentry positions. By this means information on 
enemy encroachment over a wide area may be 



Figure 53. Anti-infiltration set, Model No. 1, set 

up for operation. 

imparted to the most advantageous point with¬ 
out danger to personnel. Furthermore, the per¬ 
sonnel required may be greatly reduced. The 
ability to distinguish particular sounds and to 
determine their nature is more desirable than 
determining the direction from which they 
came. The advantage of binaural listening for 
anti-infiltration purposes lies in its improved 
discrimination and characterization of sounds, 
especially in the presence of random back¬ 
ground noise. 

Method Employed. The anti-infiltration lis¬ 
tening system is composed of two separate, 
single-direction transmission channels, each 
consisting of a microphone, an amplifier, and 
one receiver of a headset. The phase and fre¬ 


quency characteristics of the two channels are 
made identical, within small tolerances. The 
volume matching is part of the field calibration 
procedure. The/ two microphones are so located 
that they respond in practically the same man¬ 
ner as the human ears, each transmitting its 
signal to a corresponding ear of the listener. 
The effect of this arrangement is to transplant 
the listener aurally to the position of the micro¬ 
phone “head.” 

General Requirements. The idea of accurate 
location binaurally was never considered be¬ 
yond the requirement that the binaural system 
have as satisfactory localization as the unaided 
ears. It was believed desirable that in the anti¬ 
infiltration device the microphone head be di¬ 
vorced from the amplifier to facilitate ease of 
installation and to reduce the expendability of 
the apparatus. This would permit the amplifier 
to be located at the listening station, thus sim¬ 
plifying the problem of operation and repair. 

For anti-infiltration purposes the listening 
device should be capable of converting a wide 
range of sound levels into a narrow range of 
listening levels. This demand stems from the 
need to maintain relatively high listening levels 
for weak signals in order to improve their dis- 
cernibility and to overcome the effects of possi¬ 
ble high extraneous noise levels at the listening 
station. It was, therefore, necessary to incor¬ 
porate high gain and a relatively large amount 
of compression in the amplifier. 

Because the maximum output should be lim¬ 
ited to a value which satisfactorily protects the 
listener’s ears against unexpected loud sounds 
arriving at the microphones, some form of vol¬ 
ume limiting should be incorporated. 

A substantial reduction in frequency range 
over that used in the outpost system was found 
to be satisfactory. 

Operating conditions demanded a microphone 
unit that was lightweight, portable, rugged, 
and sealed. The microphone head being the 
most expendable portion of the system, it was 
desirable to use relatively cheap microphone 
elements. A transducer usable both as a micro¬ 
phone and a receiver would materially reduce 
the problem of spare parts and servicing. 

The microphone head should be designed to 
be light and compact, and to provide the best 


CONFIDENTIAL 






166 


GUN RANGING AND LOCATING SYSTEMS 


possible binaural properties. Previous experi¬ 
mental results indicated that: (1) the baffle 
between the microphones should approximate 
the effective shadowing afforded by the human 
head; (2) the microphones should be spaced 
apart the approximate distance of human ears, 
an exaggeration of 10 per cent in separation 
being satisfactory. 

Preliminary Anti-Infiltration System. The 
Western Electric type HA-2 receiver was se¬ 
lected for the first anti-infiltration set devel¬ 
oped. The head, made of Celotex, was triangular 



Figure 54. Anti-infiltration set, Model No. 2, set 

up for operation. 

in shape, so designed as to be collapsible. The 
frequency response of the amplifier covered the 
range from 60 to 5,000 c; the response range 
of the receiver fell well within these limits. The 
required gain and compression were obtained. 
The maximum power output was limited to 6 
milliwatts. Equalization of the gain and phase 
in the two channels was made possible. The 
first model is shown set up for operation in 
Figure 53. 

Specific Military Requirements. The first 
model of an anti-infiltration system was given 
field tests at Fort Bragg, North Carolina. Rep¬ 
resentatives of the Field Artillery Board ob¬ 
served this demonstration and were favorably 
impressed. The Board believed, however, that 
certain improvements should be made. They 
recommended the development of a preliminary 
combat model, provided the following charac¬ 


teristics could be obtained: (1) decreased size 
and weight; (2) increased protection to the 
listener’s ears against unexpected loud noises; 
(3) provision for multiple sentries for greater 
coverage with one operator; (4) suitable water¬ 
proofing and ruggedness of all parts, especially 
transducers; (5) a better microphone from the 
standpoint of frequency response. 

Second Anti-Infiltration System. An anti¬ 
infiltration set fulfilling the above requirements 
was developed and a preproduction model was 
completed just prior to the termination of the 
contract. In addition to very much reduced size 
and weight, this model was equipped with: 

1. An improved form of automatic volume 
control (compression) ; 

2. A volume limiter holding the power output 
to 2 milliwatts but giving usable power for 
input signals in the range from 6XlO~ n to 
6xl0 -5 milliwatt; 

3. A suitable switching arrangement for lis¬ 
tening at will to any of three binaural sentry 
stations; 

4. A sturdy waterproof head of sheet alumi¬ 
num with glass wool covering; 

5. The Marine Corps CW-59505 receiver unit, 
which is waterproof and may be used as a re¬ 
ceiver or (with an equalizing system) as a 
microphone with a response between 100 and 
4,000 c. This model, set up for operation, is 
shown in Figure 54. The total weight of the 
equipment shown is 21 lb. This set, completed in 
August 1945, was not submitted to the Field 
Artillery Board because of termination of the 
contract, but tests indicated highly satisfactory 
performance. 

5 62 Analysis of Field Records 18 ’ 19 

Analysis of field records from the western 
European front was undertaken in two differ¬ 
ent instances. 

In the first instance the analysis was carried 
out at the request of the Field Artillery Board, 
which furnished some field records that had 
yielded a doubtful interpretation at the time 
they were taken. The Division acted here in a 
consulting capacity. The ballistic-burst method 
of determining the line of flight of a shell from 
the ballistic and shell-burst waves had recently 


CONFIDENTIAL 




ACCESSORY PROJECTS 


167 


been developed. It was thought that application 
of this method, together with the experience 
members of the Division had had in distinguish¬ 
ing ballistic from gun waves, might make fur¬ 
ther analysis of these records possible. Should 
such prove to be the case, the conclusion would 
be that the ballistic wave should, when possible, 
be used in future field analysis of gun records. 

In the second instance the Division requested 
certain field records for the purpose of analyz¬ 
ing them. It was hoped that such an analysis 
might be helpful in connection with a study 
which members of the Division were making of 
errors in gun ranging due to meteorological and 
terrain effects (see Section 5.5). 

Use of the Ballistic Wave in Gun-Ranging 
Analysis 

Data Supplied. A letter of December 14, 1944, 
from the Field Artillery Board included the data 
on a series of nine acoustic locations made in 
France and Germany. Of these nine locations, 
three had already been confirmed by other 
methods, either by occupation of the territory 
or aerial reconnaissance. The films for the six 
unconfirmed locations were included for further 
analysis. The sound records supplied were taken 
by the 13th Field Artillery Observation Bat¬ 
talion operating in the vicinity of Vicht, Ger¬ 
many, during the period November 5-9, 1944. 
Two straight bases, each containing six micro¬ 
phones, separated by 4 sound-seconds, were 
used. 

Procedure. In carrying out the analyses the 
procedure was generally as follows :• 

First the traces produced by each microphone 
were studied, noting times of all breaks. Then 
all such data were treated as if the waves pro¬ 
ducing the breaks might have been ballistic, 
using the earliest time of arrival as the basis 
from which all time differences were computed. 
These computed time differences, corrected for 
temperature but not for wind, were then con¬ 
verted to meters and, with these distances as 
radii, circles were drawn about the correspond¬ 
ing microphone positions. Smooth curves tan¬ 
gent to families of these circles represent ap¬ 
proximately the traces of the various possible 
waves on the ground and, therefore, indicate 
which breaks belong together. 


The shape of the trqce obtained by treating 
the data as described may give additional in¬ 
formation as to the nature of the sound source. 
Curves obtained in this manner from ballistic 
waves, gun waves, and shell waves will, in gen¬ 
eral, be either parabolas or circles. It may be 
stated that when the curve is a parabola, the 
sound source causing it must have been a bal¬ 
listic wave. If, however, the curve is indistin¬ 
guishable from a circle, it may correspond to 
a wave from a stationary source such as a gun 
or shell burst, or to a ballistic wave with an 
essentially circular ground trace. The shell- 
burst wave could usually be identified by its 
much later arrival time, whereas the gun wave 
and ballistic wave could often be distinguished 
by making standard plots of each on the M-l 
board or its equivalent and examining the re¬ 
sultant cat’s cradles. Each type of wave gives 
a characteristic cat’s cradle. 

The next step in the analysis was to deter¬ 
mine whether the shell burst was in any way 
correlated with the gun. The gun was plotted 
by the standard method. The shell burst was 
also plotted in the same manner and was located 
in one or the other of two symmetrical positions 
in front of and behind the base line. (The sound¬ 
ranging operator in the field probably knew 
which of these two positions was the correct 
one by means of data not forwarded to the 
Laboratory.) A correlation of the time of flight 
and the range from gun to burst for each of 
these two particular locations should indicate 
which position is correct. The line of flight of 
the shell may also be determined for a given 
shell-burst position and ballistic wave. This line 
of flight should pass through or near the gun 
location, if the interpretation of the various 
waves is correct. 

It may arise that more than one gun, ballistic, 
or shell-burst wave is present, further compli¬ 
cating the analysis, but the method outlined was 
generally successful even in such cases. In cer¬ 
tain cases it was even possible to postulate the 
type of gun firing from its range and shell 
velocity. 

Results and Conclusions. The six films were 
analyzed. Five of them yielded definitely val¬ 
uable results; the sixth yielded indications 
which would require corroboration by further 


CONFIDENTIAL 



168 


GUN RANGING AND LOCATING SYSTEMS 


data before actual use. The analyses were made 
without the assistance of much auxiliary data 
which would obviously have been known to an 
observer in the field, such as the general target 
area and probable type of gun and shell. Such 
data might enable an observer to decide between 
some of the alternatives which presented them¬ 
selves. Much of the information obtained by 
this analysis could have been determined by 
inspection in the field had the sound-ranging 
bases been equipped with microphones having 
an upper frequency limit of 50 or 60 c (unmodi¬ 
fied T-21-B microphones were used). The 
method developed for the analyses yielded, in 
several cases, quite different diagnoses from 
those obtained by inspection in the field. It was 
believed some features of this method are suffi¬ 
ciently simple and yield sufficient information 
of practical value to be considered for stand¬ 
ardization. (Use of the ballistic and shell-burst 
waves for determining the line of fire and 
caliber of the gun has since been incorporated 
into the War Department Field Manual FM 6- 
120 .) 

Meteorological and Terrain Effects in 
Gun-Ranging Analysis 

As a result of having seen some British data 
which was insufficient to do more than to indi¬ 
cate a trend, the Division requested further 
data to investigate the question of the relation¬ 
ship between the type of error—range or line— 
predominating in sound ranging and the type 
of meteorological conditions existing at the time 
the measurements were made. 

Data Supplied. The analysis was carried out 
on the data of Army Ground Forces Reports 
C-Misc-29 and 30, ETO, which were forwarded 
via the Field Artillery Board on December 22, 
1944. The data were taken by the 3rd, 7th, 8th, 
and 16th Field Artillery Observation Battalions 
and included such items as time of firing, loca¬ 
tion of base, sound-ranging location, sound 
metro message, comparison of sound location 
with adjusted coordinates of enemy batteries, 
comparison of photographic interpretation lo¬ 
cations with confirmed sound locations, flash¬ 
ranging location, and report on effect of fire. 
Not all these items were supplied in each case. 
Unfortunately, the equivalent of a map survey 


position of the gun appears to have been un¬ 
known for a large number of the shots for 
which data were included, making the data in¬ 
adequate for carrying out the type of analysis 
originally proposed. However, the data were 
sufficient to yield information of value to sound¬ 
ranging officers. 

Conclusions. No clear relationship between 
the meteorological conditions present and the 
type of error—range or line—predominating 
in sound ranging was revealed by the analysis 
of the data. The desirability of analyzing data 
better adapted for this study was pointed out. 

A study of the data indicated clearly the ne¬ 
cessity for frequent meteorological messages 
when conditions are changeable, but yet not so 
unstable that the application of meteorological 
corrections is unwarranted. When conditions 
are stable, the present intervals are satisfac¬ 
tory. 

If a sound source is located in or immediately 
behind a village, larger errors should be ex¬ 
pected than for sources in the open. 

Two methods of computing effective winds 
for sound ranging were outlined, both of which 
use the present standard weighting but obtain 
a more accurate effective wind by employing 
vector rather than scalar addition. A compari¬ 
son of the values of the effective wind computed 
by the new methods sometimes gave results 
widely different from those obtained by the 
standard method, although in many cases they 
were nearly identical. 

Recommendations. The following recommen¬ 
dations should be considered tentative in view 
of the small amount of data upon which they 
are based: 

1. The question of the relationship between 
the type of error—range or line—predominat¬ 
ing in sound ranging and the type of meteoro¬ 
logical conditions existent at the time the meas¬ 
urements were made should be studied further. 

2. Meteorological messages should be taken 
more frequently when the meteorological con¬ 
ditions are changing rapidly. 

3. Caution should be used in assessing the 
accuracy of sound locations which indicate that 
a gun is in the vicinity of a village, since rela¬ 
tively poor sound locations resulted in most 
cases where the gun was located either in or 


CONFIDENTIAL 



ACCESSORY PROJECTS 


169 


near a village so that the sound path is over 
buildings. 

4. The method of calculating the effective 
wind for sound ranging should be investigated. 

5 . 6.3 p r 0 p 0ge( j Method of Sound Ranging 

Eliminating Meteorological Corrections 23 

In the standard method of sound ranging it 
is necessary to correct the arrival times of the 
sound wave at the microphone base for the 
effects of the existing meteorological conditions. 
In standard practice these meteorological cor¬ 
rections are computed empirically from pilot- 
balloon ascensions and from ground tempera¬ 
ture measurements. This procedure immediately 
gives rise to certain possibilities of error, for 
the following reasons: 

1. The pilot-balloon ascensions and ground 
temperature measurements are ordinarily made 
at intervals of about one hour, on the assump¬ 
tion that the meteorological structure of the 
atmosphere varies slowly during this interval, 
and also that the wind and temperature vary 
with height uniformly over the region in which 
sound-ranging measurements are made. Actu¬ 
ally, it appears highly probable that neither 
of these assumptions is true. 

2. Even if the existing meteorological struc¬ 
ture has been determined accurately, it is still 
necessary to reduce this structure to an equiva¬ 
lent wind and temperature which are uniform 
with height. The procedure for doing this is 
entirely empirical (see Section 5.3). 

The purpose of this work was to formulate 
a new method of sound ranging which would 
require no corrections for the effects of wind, 
temperature, or curvature of the wave front. 
In addition to its possibilities as a method of 
sound ranging, such a method could also be 
used as a very powerful tool in the measure¬ 
ment of effective meteorological conditions and 
of their effects on the shape of the wave front. 

Principles Involved. The development of this 
method was started as a result of a British 
report forwarded by the OSRD Liaison Office 
(Loga 6914, Appendix II). This report gives 
the basic theory of this method, employing the 
method of least squares. The new method uses 
a two-dimensional microphone array and re¬ 


quires six or more microphones. By determin¬ 
ing the time of arrival of the sound wave at 
each of these microphones with respect to an 
arbitrary time zero, it would be possible to 
compute directly the position of the sound 
source with respect to a known position in the 
target area (see Figure 55). There are six 
variables concerned: 

1. The distance x in the X direction of the 
actual source from the known point; 

2. The distance y in the Y direction of the 
actual source from the known point; 

3. Value of the effective wind component in 
the X direction (a) ; 

4. Value of the effective wind component in 
the Y direction (/ 3 ) ; 

5. Still-air sound velocity for the existing 
temperature; 

6. Total time of travel of the sound from the 
source to the microphone. 

It will be seen that time measurements at 
each of six or more microphones would furnish 
sufficient data to eliminate the last four of these 
variables and solve for x and y. 

If this method were used as a tool for the 
study of effective meteorological conditions and 
of their effects on the shape of the wave front, 
the three variables which would be solved for 
are: 

1. Component of the wind in the X direction; 

2. Component of the wind in the Y direction; 

3. Still-air sound velocity. 

From these results it would be possible to com¬ 
pare the actual effects of the meteorology exist¬ 
ing during the sound travel from source to 
microphone array with that computed by the 
present method or any proposed improvement 
thereof. 

Equipment Required. To apply this method 
in the field, the following equipment would be 
needed: eight sound-ranging microphones to 
form the array shown in Figure 56; one oscillo¬ 
graph with eight recording elements, such as 
the standard GR-3-C recorder currently used 
by the U. S. Field Artillery Observation Bat¬ 
talions; plotting board for determining loca¬ 
tions; and a table giving values of certain 
constants involved, as computed for a series of 
reference points selected in the area within 
which enemy guns might be expected. 


CONFIDENTIAL 



170 


GUN RANGING AND LOCATING SYSTEMS 


Equipment Eliminated by this Method. The 
use of this method would eliminate the follow¬ 
ing personnel and equipment at present neces¬ 
sary in the standard method of sound ranging: 
meteorological section and equipment necessary 
to determine effective sound-ranging meteoro¬ 
logical conditions, charts for determining the 
meteorological corrections for the effects of 
wind and temperature, and charts for the de- 



Figure 55. Method of sound ranging eliminating 
meteorological corrections. 


termination of corrections for wave front 
curvature. 

Sound-Ranging Procedure. The actual field 
procedure to be used in the application of this 
method is outlined below. 

The arrival times, t lf t 2y • • • t 8 of the sound 
wave at each microphone are read from the 
oscillogram with reference to an arbitrary zero 
time. 

It is necessary to make a rough determina¬ 
tion of the location of the source, so that the 
reference point closest to the actual gun location 
can be selected. 

The values of the quantities x and y can be 
calculated as follows. 

= -d.11^1 “I - A21 £2 4“ A.31^3 -(- A41^4 “L-A51^5 ~|—A.61^6 4“ A 71^7 4“ A sits 
y = Al2^1 4" A22^2 4" A32^3 4“ A42^4 4”A52^5 4" A62^6 4~A 72^7 4“ A 82^8 4~ B2 

where the A’s and B’s corresponding to the ref¬ 
erence point selected are obtained from the 
table described in Section 5.6 under Equipment 
Required. The actual calculation in the field 
is then reduced to the multiplication of sixteen 
A values by the appropriate t’s and the sum¬ 
mation of the resulting products to give the 
x and y values, as indicated in the above equa¬ 
tions. 

The actual location is then obtained by plot¬ 


ting these x and y increments from the selected 
reference point. This location gives the most 
probable position of the sound source and takes 
into account the effective wind and tempera¬ 
ture, although these quantities are not solved 
for explicitly. 

Investigation of Major Meteorological Struc¬ 
ture. It has been indicated that considerable 
inaccuracy may be introduced into sound-rang¬ 
ing measurements by the method of determin¬ 
ing the effective sound-ranging meteorological 
conditions as based on pilot-balloon ascensions. 
From an array of the type shown in Figure 56 
it would be possible to determine the most 
probable value of the two components of the 
instantaneous effective wind and the most prob¬ 
able value of the instantaneous still-air sound 
velocity. By taking a series of sound records 
for sources at various ranges and azimuths 
with this two-dimensional array and simultane¬ 
ously determining the effective sound-ranging 
meteorology by pilot-balloon ascensions, it 
would be possible to determine, by a compari¬ 
son of the results of these two methods of 
making meteorological measurements, the fol¬ 
lowing: (1) whether an effective wind-and- 
temperature structure exists which is constant 
over any appreciable area and time; (2) if 
such an effective meteorological structure does 
exist, whether the present method of determin¬ 
ing this structure by pilot-balloon ascensions 
and ground temperature measurements gives 
a sufficiently accurate evaluation of it; (3) if 
the present method of determining sound¬ 
ranging meteorology is unsatisfactory, whether 
some improved method of weighting meteoro¬ 
logical data can be devised which will give a 
better approximation to the effective conditions 
as measured by the two-dimensional array. 

Investigation of Micromet. In addition to this 
problem of the major meteorology, there are 
also fluctuations from these average conditions 
which can occur in a period of time of the 
same order of magnitude as that during which 
the sound propagation occurs (see Section 5.5). 
This two-dimensional array presents a method 
of determining whether these rapid fluctua¬ 
tions are of importance in their effect on 
sound-ranging measurements. 

Thus, if one of these two-dimensional micro- 


CONFIDENTIAL 






ACCESSORY PROJECTS 


171 


phone arrays could be set up with a reference 
point at a known position with respect to it, then 
if a series of shots were set off at this refer¬ 
ence point and the locations determined by the 
method described above, it is seen that the 
values of x and y should be zero. Any deviation 
could be attributed to the fluctuations in the 
meteorological conditions. 

As an alternative, the three effective 
meteorological factors—(1) the component of 
the wind along the sound path, (2) the com¬ 
ponent of the wind at right angles to the sound 
path, and (3) the effective velocity of sound 
over the sound path—could be determined for 
this fixed position of the source. If shots were 
fired every minute or every two minutes, the 
variation of these factors with time could be 
determined. 

Summary of Results. No actual field tests of 
the method were made, although a number of 
theoretical cases were calculated. The method 
has the following applications: (1) as a prac¬ 
tical field method of sound ranging without the 
use of meteorological corrections; (2) as a 
research method of determining the effective 
major meteorological structure for sound 
ranging; (3) as a research method for the in¬ 
vestigation of the effect of fluctuations in the 
meteorological conditions upon the accuracy of 
sound ranging. 


5 - 64 Sphinx 24 

In the war in the Pacific the enemy fre¬ 
quently took to hiding in caves, making the 
locating of such caves an important problem. 
As a satisfactory solution had not been found, 
the Armed Forces requested the Division to 
investigate the possibility of locating such caves 
by acoustic means. Work on this project was 
abandoned with the ending of World War II, 
but results of a preliminary nature were ob¬ 
tained. 

Principles Involved. A cave is an enclosure 
with an opening to the air; as such it will be 
resonant at certain acoustic frequencies which 
are a function of the cave’s dimensions. Conse¬ 
quently, if a complex sound wave containing 
energy in the correct frequency regions—such 


as might result from ^n explosion—impinges 
on the opening of thq cave, these resonant 
frequencies will be excited and radiate sound 
selectively. It was the purpose of this investiga¬ 
tion to determine if this reradiated sound had 
sufficient intensity and was sufficiently unique 
in character to be distinguishable from nor¬ 
mally reflected sounds. If this was so, it would 
be possible to sound range on caves by setting 
off a blast in their neighborhood. 

Theoretical Considerations. The results of a 
theoretical study of this problem indicated that 
when the air in a cave is actuated by an external 
sound, low-frequency sounds are generated by 
the action of the cave as a Helmholtz resonator, 


ms 

M7 

> — 

5 M 4 

M3 

AREA OF 
ENEMY 
ACTIVITY 

MICROPHONE 


ARRAY 


M 6 « 


'M2 

M 5 < 

i —< 

>M1 


Figure 56. Two-dimensional microphone array. 


and higher-frequency sounds are generated by 
internal reflections (reverberations). Intensity 
considerations favor working at low frequen¬ 
cies. Assuming the actuating sound to be an 
impulse produced by an explosion, it was com¬ 
puted that the sound should be of sufficient 
duration to excite the air in the cave, but not 
so long as to make the identification of the 
sound from the enclosure difficult due to 
troublesome reflections. The possibility of excit¬ 
ing the cave by means of an earth-borne wave 
was also considered worthy of experimental 
investigation. 

Equipment. Two enclosures were used. One 
was a 3-ft cubical plywood box with a 20-in. 
diameter hole cut in its face. This box was 
used mainly to test equipment and to give 
an indication of the factors which might be 
expected in later field experiments. The second 
enclosure investigated was a concrete bunker, 


CONFIDENTIAL 











172 


GUN RANGING AND LOCATING SYSTEMS 


whose inside and outside dimensions are indi¬ 
cated in Figure 57. (The openings at B and C 
were closed in the final tests.) While this is not 
properly a cave, a sufficient indication of the 
feasibility of sound ranging on caves should 
be obtainable by a study based on this bunker. 

The microphones used in these tests consisted 
of T-2 and WE type 633 microphones. The 
T-2 microphone is the crystal type developed 

PLAN OF BUNKER 


i.l FT 


I-1 

I I 
I I 
J l 


T 

h- 

I 

I 

1 


1~ 


-T 4.1 ft 

±i£i 


Y 5.8 FT 


ELEVATION 



Figure 57. Plan of bunker. 


by the Division (see Section 5.4). It has a low- 
frequency cutoff of about 3 c. For the frequency 
range from 50 to 500 c the Western Electric 
type 633, a low-impedance, dynamic type of 
microphone, was used. 

Special types of amplifiers were required for 
each of the microphones. It was necessary to 
design the units with a volume limiter having 
a very sharp cutoff (1) to avoid damage to the 
oscillograph strings from signals with too much 
amplitude, and (2) so that the large signals 
from the direct sound of the bursts would not 
block the amplifier for more than 50 milli¬ 
seconds. 

A rapid-record oscillograph (the six-string 


galvanometer type using a 70-mm photographic 
paper for recording the traces) was used dur¬ 
ing these tests. 

Procedure and Results. Field experiments 
were performed in which an M-l rifle was 
fired 40 ft in front of the plywood box, first 
with the hole open and then with it closed. With 
the hole open, the oscillogram showed the 
sound of the rifle followed by the waves re¬ 
flected from the box and a reverberant tail. 
With the hole closed, the reverberant tail was 
missing. Thus it was possible to detect the 
“cave” reverberations (about 180 c) for dis¬ 
tances from 50 to 100 ft in front of the open 
box. However, with this sound source it was 
not possible to excite the frequency of the box 
sufficiently as a Helmholtz resonator (43 c in 
this case). 

Subsequent tests were made with the bunker, 
which approximated a full-sized cave. Com¬ 
monly available explosive sounds were used. 
The oscillograms showed evidence of resonance 
phenomena when the microphones were 3 ft 
in front of the entrance but gave no indications 
of resonance when located 30 ft or more away 
(see Figure 58, in which A, B , 1, 2 , and 3 
indicate various microphone positions). It was 
evident that both the reverberation and reso¬ 
nant frequencies were insufficiently excited to 
produce an intensity detectable at any signifi- 



Figure 58. Survey of bunker, microphones, and 
shot point. 


cant distance from the cave. Reflection from the 
surrounding terrain and extraneous objects was 
of sufficient intensity to mask completely the 
cave resonance sounds. It is believed that this 
may be due to the small energy content of the 
explosion in the desired frequency range, i.e., 
below 50 c, and that work should be initiated 


CONFIDENTIAL 











































PRESENT STATUS OF THE SUBJECT 


173 


to obtain explosive sources having high energy 
content in the frequency range from 5 to 50 c. 

The problem was investigated only superfi¬ 
cially in the short time available. Its importance 
may warrant further investigation. 


General Consulting 

In a few instances, problems were under¬ 
taken by the Division at the specific request of 
the Armed Forces. This was the case with 
work on the ballistic-burst method, the analysis 
of certain field records, and Sphinx. In each 
of these cases some work on the subject had 
already been done by others, and the Division 
was requested to continue the investigation. 

Frequent conferences attended by members 
of the Division and military personnel were 
held. At such conferences the military men 
presented some of their problems, and these 
were then discussed. Members of the Division 
always considered themselves available for call 
to such meetings. Many of the particular proj¬ 
ects discussed in this report originated in this 
manner. 

During many of the field tests of methods 
and equipment which were conducted at Fort 
Bragg, North Carolina, Fort Sill, Oklahoma, 
and Quantico, Virginia, members of the Divi¬ 
sion worked closely in the field with military 
personnel, both officers and enlisted men. Many 
friendly contacts were established. In this way 
members of the Division learned at first hand 
some of the problems encountered by the men 
in the field. Often the Division members were 
able to help the military personnel to under¬ 
stand the sound-ranging equipment better and 
to use it more effectively. The liaison officers to 
the Duke project have emphasized that this 
education of sound-ranging personnel was one 
of the most important contributions rendered 
by the Division. 


57 PRESENT STATUS OF THE SUBJECT 

The military importance of sound ranging 
has long been accepted. The work described in 
this report should make it clear that there was 


a definite need for research in this field, and 
that this research produced valuable results. 
However, it should be emphasized that the pos¬ 
sibilities for research along this line were by no 
means exhausted, nor were all the individual 
projects undertaken followed through com¬ 
pletely. Many were left incomplete at the ter¬ 
mination of the contract. Others were aban¬ 
doned to concentrate on other work of greater 
urgency, or because they would not yield re¬ 
sults of immediate use in a war nearing its end. 
The majority of such projects deserve further 
study, both from the purely scientific and from 
the military standpoint. 

Pure research on fundamentals is a necessary 
background to the best results in all practical 
development work. Such research requires a 
long-sustained program, usually impossible dur¬ 
ing the urgency of a war period; it must be 
continued during times of peace. 

Scientific advances predetermine new types 
of warfare, making new research on the mili¬ 
tary applications of a given field always neces¬ 
sary. Sound is no exception. For example, rocket 
projectiles require a different method of sound 
ranging from that applicable to ordinary guns. 
Again, an underground type of defense presents 
a problem entirely different from that of usual 
ground warfare. 

Research on a particular problem frequently 
suggests new lines of development quite differ¬ 
ent from that of the original problem. Thus new 
methods of offense as well as of defense may 
suggest themselves, and certain methods now 
in use may be found not worth the effort and 
expense they entail. 

It is advantageous if new methods and equip¬ 
ment can be exhaustively tested and studied in 
non-combat action, where failures or mistakes 
can be discovered without costly results. 

Types of Bases. The study of different types 
of bases showed that the choice in a specific 
instance must always depend on circumstances. 
It was never universally agreed whether a one¬ 
dimensional or a two-dimensional base was pref¬ 
erable. Further investigation of this problem 
is required. 

Microphones. The work of the Division 
showed that there was a lot of room for im¬ 
provement in the types of microphones used in 


CONFIDENTIAL 



174 


GUN RANGING AND LOCATING SYSTEMS 


military sound ranging. Response, durability, 
and portability were improved; it is quite pos¬ 
sible that other improvements could be made 
through further research along this line. For 
example, at the termination of the work on the 
T-2 lightweight crystal microphone, it was pos¬ 
sible to recommend additional changes, such as 
a more waterproof type of plug. Time did not 
permit these suggestions to be investigated. 

Binaural Outpost and Binaural Listening. 
The binaural outpost system developed by the 
Division was tested by the Field Artillery Board 
and found to perform the functions for which 
it was designed. However, it was never tested in 
combat because it was decided that it was more 
important to transport other equipment, deemed 
more necessary, to the fighting fronts. The pos¬ 
sibilities of this outpost system should be more 
fully investigated. The binaural anti-infiltration 
set reached the stage of a preproduction model 
and deserves further development. Further im¬ 
provements were suggested by the Division, 
such as tropicalization and the acquisition of a 
cheap, rugged microphone with a flat response. 
The equipment should be given thorough field 
tests. 

Computing. New systems require new meth¬ 
ods of computing. There are many possible 
methods that could be applied to a given system, 
and only investigation can show which is most 
practical. It was felt by the Division that the 
nomogram was a definite contribution, but be¬ 
cause of the termination of World War II, its 
possibilities were never fully realized by the 
Field Artillery. Further education in its effi¬ 
cient use seems necessary. 

Ballistic-Burst Method. The advantages of 
this method were realized by the Field Artillery, 
and it is now included in Field Manual (FM 
No. 6-120). Its further application to ranging 
on rocket projectiles and new types of guns is 
worthy of attention. 

Templates, Grids, Cases, etc. The trace-read¬ 
ing template proved definitely helpful. The 
Lucite plotting grids and accessory equipment 
(Lucite fans, microphone base, etc.) were very 
favorably received by the Field Artillery Board, 
but termination of World War II prevented 
their being tested thoroughly. Various types of 
cases were designed for these grids, as well as 


for the nomogram, Dodar, and microphones. The 
case for the Dodar incorporated several im¬ 
provements over existing types of cases for 
military equipment, especially in ruggedness 
and watertightness. The features of this design 
should be applicable to other types of portable 
instruments. 

Dodar. The value of the Dodar was shown in 
the Iwo Jima campaign. However, in subsequent 
combat use, it did not make as favorable a 
showing. It was felt by those most familiar with 
it that it was not handled in the most efficient 
way. This was partly because time did not per¬ 
mit sufficient education of the military person¬ 
nel in its use and maintenance. A new method is 
often not generally accepted until it has proved 
itself. This should now be possible. Further, as 
mentioned earlier, a new ultralightweight Dodar 
system was developed near the end of the 
Division’s contract. This system was demon¬ 
strated in its experimental stage and proved 
worthy of further development. The increased 
mobility of modern warfare emphasizes the 
need for a system such as that furnished by the 
Dodar. 

Sphinx. Time did not permit anything but 
a preliminary investigation of this problem. The 
increased tendency in modern warfare to go 
underground puts great importance on the lo¬ 
cation of enemy caves, dugouts, and other 
shelters. The fact that resonance vibrations in 
such air cavities can be detected at all, even 
though at only short distances from the en¬ 
trances is significant; it suggests further in¬ 
vestigation, using different sound sources 
(preferably those with more energy concen¬ 
trated in a low-frequency band) and different 
types of receivers. The possibility of using 
earth-borne waves is also considered worthy of 
further study. 

Meteorological and Terrain Effects. The im¬ 
portance of meteorological effects in sound 
ranging should be clear. As has been pointed 
out, these effects may be divided into those pro¬ 
duced by the major meteorological structure 
and those due to micrometeorological effects. 
Each must be considered serious and, at times, 
very troublesome. It was felt by the Division 
that the greater emphasis placed by it on micro- 
meteorological effects was a very important con- 


CONFIDENTIAL 



PRESENT STATUS OF THE SUBJECT 


175 


tribution to the subject of sound ranging. How¬ 
ever, it has been pointed out that this study 
could only be looked upon as a beginning, and 
that an exhaustive investigation of the subject 
must be undertaken as a long-term research 
program. It was recognized that the greatest 
possibility of reducing meteorological errors 
may be not in seeking improvements in present 
sound-ranging methods, but in finding radically 
different ones which might utilize other, now 
unused aspects of transmission phenomena. An 


example of this is the proposed method of sound 
ranging discussed in Section 5.6. The value of 
the theoretical and experimental work on sound 
propagation in air above a boundary surface 
should also be stressed, as the results of this 
work suggest a modification of existing theories 
and interesting future applications. It is hoped 
that this fundamental physical research will be 
continued during peacetime, not only for its 
military value but also for what it may add to 
scientific knowledge. 


CONFIDENTIAL 



Chapter 6 

INFRARED GAS DETECTORS AND ANALYZERS 

By Clark Goodman a 


61 INTRODUCTION 

A lthough detection, identification, and 
. measurement of gases by study of their 
spectral absorption bands have been laboratory 
procedures for many years, development of 
portable and easily operated apparatus using in¬ 
frared absorption bands of gases as the means 
of detection has been accomplished only re¬ 
cently. Infrared gas detectors may be either 
nonselective or selective. For either type of 
apparatus, a response time of a few seconds is 
obtainable. 

The military interest in gas detectors arises 
from certain field situations which involve 
poisonous gases as weapons, and certain ventila¬ 
tion problems of closed equipment which in¬ 
volve poisonous gases as incidental products 
of gunfiring or other operations. It is evident 
that any attack on these problems requires 
suitable means for detection and for measure¬ 
ment of concentration of the particular gases 
involved. The principle of infrared detection of 
gases was selected for development because of 
its inherent rapid response and its high sensi¬ 
tivity and high selectivity for certain gases, 
particularly carbon monoxide (CO) and carbon 
dioxide (C0 2 ). Furthermore, the fact that its 
output is electric provides the possibility of 
using a direct-reading meter or chart recording 
of the final indication. All the possibilities of 
this type of apparatus have not been exploited, 
but equipment now available appears to be par¬ 
ticularly useful for ventilation studies and for 
certain laboratory problems in testing of gas 
masks. It certainly could be used as a device to 
warn of the accumulation of dangerous gases 
in closed equipment; in fact, it has been used in 
tanks and also to detect C0 2 in submarines. It 
does not, however, appear applicable to field use 
as a detector of war gases. 

Nonselective types are designed for detecting 
and measuring a small quantity of a gas when 

a Technical Aide, Division 17. 


no other absorbing gases are present, or when 
only one constituent of a gas mixture is chang¬ 
ing concentration, and will detect a large num¬ 
ber of gases to limits of a few parts per mil¬ 
lion. These detectors are particularly useful for 
study of the breakdown of gas masks, measure¬ 
ments of water-vapor concentrations in oxygen, 
study of the change in concentration of a 
“tracer” gas in ventilation work, etc. 

Selective-type instruments are designed for 
detecting and measuring concentrations of C0 2 
or CO (or certain selected gases) in the pres¬ 
ence of other absorbing gases. They are made 
in such a way that they will ignore changes in 
concentration of most gases other than the one 
being investigated. For example, the carbon 
monoxide in the breathing zone of a gunner can 
be correctly measured in the presence of am¬ 
monia, water vapor, carbon dioxide, oxides of 
nitrogen, and the other gaseous products of 
gunfire. Such instruments have been or may be 
used for CO analysis in tanks, determination 
of ventilation rates in tanks, survey work in 
aircraft, study of submarine atmospheres, etc. 


6 2 GENERAL PRINCIPLES OF INFRARED 
GAS DETECTORS 

Infrared gas detectors make use of the infra¬ 
red absorption bands of certain gases. These 
bands are characterized by certain frequencies 
at which the gas absorbs energy. In a simple 
way, one may think of a space filled with the 
gas as containing a great number of oscillators 
composed of masses of springs, corresponding 
to the atoms and the chemical bonds by which 
they are held together to form gas molecules. 
If radiant energy containing the proper fre¬ 
quency components passes through a space con¬ 
taining such oscillators, they will be set in 
motion. Their energy of motion is absorbed 
from those components of the entering radiant 
energy having the same frequency; therefore, 


176 


CONFIDENTIAL 



SELECTIVE GAS-ANALYSIS APPARATUS 


177 


the radiant energy leaving the space will be 
deficient in those same frequencies, giving rise 
to the absorption bands. These bands are in the 
long-wavelength range of the radiation spec¬ 
trum, which is beyond the visible range, i.e., 
the infrared (or heat spectrum). All dipole 
molecules (i.e., those composed of two unlike 
chemical atoms, such as CO and HC1) have 
infrared absorption bands; gases composed of 
like chemical atoms (such as H 2 and 0 2 ) do not. 

The following brief list indicates some of the 
combinations which can and which cannot be 
selectively measured in devices of this type by 
their infrared absorption. 

A C-H gas can be detected in the presence of 
CO, C0 2 , S0 2 , and the elemental gases, such as 
0 2 , H 2 , and N 2 . 

CO can be detected in the presence of C0 2 , 
H 2 0, NHo, C-H gases, elemental gases, and most 
others. 

Similarly, C0 2 can be detected in the presence 
of CO, H 2 0, NH 3 , C-H gases, and most others. 

Gases which have overlapping bands (one of 
which, therefore, cannot be selectively detected 
in the presence of others) include NH 3 and H 2 0; 
H 2 0 and the C-H gases; NH 3 and the C-H 
gases; one C-H gas in another (i.e., ethane can¬ 
not be selectively detected in methane). 

Investigations of infrared spectra and any 
application of their properties involve the use 
of direct radiation detectors. The spectral range 
is entirely outside that in which visual obser¬ 
vation or photography is possible; neither can 
such things as photoelectric cells or barrier- 
layer cells be used. The radiation detectors com¬ 
monly used for infrared work are radiometers, 
bolometers, or thermopiles. The applications dis¬ 
cussed here use a thermopile, which is a series 
of thermocouple junctions connected in series, 
so that their voltages add up; such a thermopile 
is responsive to extremely small amounts of 
radiation. By measuring the output voltage of 
a suitable thermopile with a sensitive gal¬ 
vanometer or with a d-c amplifier, it is possible 
to detect and measure or record the small 
changes of energy associated with changes in 
absorption in the infrared bands. 

Important practical limitations arise because 
many materials (in particular, glass) are not 
transparent to infrared radiation; consequently, 


lenses cannot be used and any focusing is usu¬ 
ally done with concave mirrors. Windows are 
commonly made of rock salt, fluorite, or lithium 
fluoride, depending on the wavelength range to 
be used, as these materials transmit much far¬ 
ther into the infrared than glass. 

6 3 SELECTIVE GAS-ANALYSIS APPARATUS 

631 Model IV 

A gas-analysis apparatus of the selective type 
was developed at Johns Hopkins University 1-5 * 7 



Figure 1. Essential parts of the infrared selective 
gas analyzer, arranged for determining CO in air. 

and placed in pilot manufacturing by the Leeds 
& Northrup Company. 6 * 7 The essential parts of 
the gas analyzer proper are shown in cross sec- 


CONFIDENTIAL 



























































178 


INFRARED GAS DETECTORS AND ANALYZERS 


tion in Figure 1. This unit of the apparatus is a 
tubular assembly containing four essential 
parts: 

1. A brass chamber containing the source S, 
which is Nichrome spiral, heated by an electric 
current. 

2. The sampling chamber, which is a Pyrex- 
glass tube with inlet and outlet branches. This 
tube is gold plated on the inside and the gold is, 
in turn, covered with an evaporated layer of 
stibnite, which protects the gold surface from 
absorption of gas and maintains the infrared 
reflectivity. 

3. The filter cell chamber, which is a Pyrex- 
glass cone with a partition down the center di¬ 
viding it into two semiconical parts. This cone, 
like the sampling chamber, is gold plated. 

4. The thermopile chamber which holds the 
differential thermopile. There are lithium flu¬ 
oride windows between source and sampling 
cell, between sampling cell and filter cell, and 
between filter cell and thermopile. Since the 
characteristics of the apparatus are critically 
dependent upon the concentration of gases in 
the filter cells, these have to be closed almost as 
completely as if they held a high vacuum. 

The electric system is shown in Figure 2. 
The output from the differential thermopile and 



Figure 2. Basic circuit of selective gas analyzer. 


balancing network is fed into a chopper ampli¬ 
fier, which, in effect, converts the d-c output 
from the thermopiles to a small a-c voltage, 
amplifies that voltage, and then reconverts the 
output back to direct current, so that it can be 
used to operate a d-c recording milliammeter 
(with full-scale deflection for 1 ma). 

The various resistances and adjustments 
which are shown are those necessary for (1) 
setting the bucking potential and thermopile 
shunt, thereby balancing the thermopiles under 


certain standard conditions, (2) setting the 
heating current through the source to a stand¬ 
ard value, and (3) applying a standard test 
signal to the amplifier. (Thermopile balance is 
quite critically dependent upon the temperature 
of the radiation source, and that source must 
be supplied from a well-charged battery and 
dropping resistor to maintain a standard po¬ 
tential.) A separate 6-v battery supplies the 
power for the chopper amplifier. 

The gas analyzer, a small flow meter to indi¬ 
cate gas flow, the associated components of the 
electric circuits, and the chopper amplifier and 
its power-supply vibrator unit are assembled 
together in a box (Figures 3 and 4) measuring 
14% in. high by 16 in. long by 9% in. wide and 



Figure 3. End view of carrying case holding the 
infrared gas analyzer and its associated electric 
equipment. 


weighing 45 lb. The complete equipment, in¬ 
cluding battery box (to hold two 6-v automobile- 
type storage batteries), analyzer, and recording 
output meter, is shown in Figure 5. 

The sensitivity of this apparatus is limited 


CONFIDENTIAL 


























SELECTIVE GAS-ANALYSIS APPARATUS 


179 


by the background noise and, therefore, de¬ 
pends upon the conditions under which it is 
used. In laboratory tests, with only mild tem¬ 
perature fluctuations and relative freedom from 
drafts, the apparatus now built will reliably 
and clearly detect CO selectively down to con- 



Figure 4. Carrying case containing the infrared 
gas analyzer and the circuit that permits electric 
measurements of heat radiation absorbed by 
selected gases. 

centrations of about 0.005 per cent, as is indi¬ 
cated by the sample chart from the recorder, 
shown in Figure 6. This apparatus has been 
designed specifically to measure CO in the range 
from 0.01 per cent to 1 per cent. It is possible 



Figure 5. Complete infrared gas analyzer, re¬ 
corder, and storage batteries. 


that under field conditions, with more exposure 
and wider temperature fluctuations, the output 
will be less stable, which may materially raise 
the lower limit. 

The present apparatus is not suitable for 


measuring CO or C0 2 concentrations greater 
than about 1 per cent, but a shorter sampling 
cell would enable the instrument to measure 
higher values. 


632 Model V 

The apparatus just described was known as 
the Model IV gas analyzer. In September 1944, 
after Edgewood Arsenal had made the neces¬ 
sary arrangements with NDRC, the Chemical 
Warfare Service placed an order with Leeds & 
Northrup for twelve gas analyzers. Production 
difficulties resulted in a practically new design, 
although the outward appearance and general 
construction of the instruments were the same 
as Model IV. One essential difference was that 
the motor-driven chopper amplifiers with the 
vibrator plate supply were no longer available. 
Consequently, General Motors Series 400 motor- 
driven amplifiers were used instead. These re¬ 
quired a somewhat different circuit and em¬ 
ployed a dynamotor as a plate supply instead 
of the previous vibrator type. The thermopiles 
used were made with bismuth versus silver 
wire. Insulating strips were mounted inside the 
battery boxes to insulate the batteries from the 
metal boxes. The operation of these instruments 
was essentially the same as the Model IV and 
were known as Model V. 


63 3 Model VI 

Both the Model IV and the Model V instru¬ 
ments were subject to changes in reading when 
the instrument was tilted. A large part of this 
effect was caused by air in the thermopile 
housing flowing from one side to the other, 
thus changing the thermal equilibrium of 
the differential thermopile. The improvements 
which were indicated in the earlier models were 
finally incorporated into an instrument known 
as the Model VI. 

The Model VI was built at the request of 
OSRD for a selective gas analyzer capable of 
satisfactory operation in a submarine and for 
other conditions where the instrument would, 
of necessity, be tilted. A program was started 


CONFIDENTIAL 














180 


INFRARED GAS DETECTORS AND ANALYZERS 


in May 1944 to build five instruments, rede¬ 
signed to meet specific requirements for sub¬ 
marines. Three of these instruments were made 
to have a range of approximately 0 to 8 per cent 
C0 2 with an accuracy of better than 0.5 per 
cent. The two remaining devices were essen¬ 
tially the same as the other three except that 
provision was made for using longer sample 


evacuating the thermopile and source housings, 
the internal pressure being reduced to 10 mm 
of Hg before sealing. The windows were of 
either LiF or CaF 2 , as called for by the particu¬ 
lar application. The source was run on regu¬ 
lated 110-v alternating current, and the poten¬ 
tial for zero adjustment produced by a thermo¬ 
couple on the source housing. A simplified am- 




Figure 6. Sample of recorder chart, showing the analyses of CO mixtures con¬ 
taining six different concentrations, 0.01-1.27 per cent. The variables recorded 
are time and milliamperes. The rapidity of sampling and instrument response 
are indicated. The deflection for 0.01 per cent is sufficient to permit the indication 
of 0.005 per cent. 


tubes. This enabled the sensitivity to be in¬ 
creased by a factor of 2 or 3. 

Inasmuch as suitable alternating current is 
available on submarines, these instruments 
dispensed with battery operation and were 
designed for 115 v. This resulted in some sim¬ 
plification of the circuit and the equipment. 
Insensitivity to tilting effect was secured by 


plifier developed by Leeds & Northrop was em¬ 
ployed. Although this amplifier did not detect 
as small a signal as the General Motors ampli¬ 
fier, it was found to be more than adequate for 
submarine application. This amplifier also op¬ 
erated on 110-v alternating current, and its 
output was indicated on a zero-to-one milliam- 
meter. For most applications, the only manual 


CONFIDENTIAL 
































































APPLICATIONS OF INFRARED DETECTORS AND ANALYZERS 


181 


control required was one dial for zero adjust¬ 
ment. 

For applications involving low gas concentra¬ 
tions, the voltage regulator did not provide a 
sufficiently stable voltage source. Also, in some 
cases a 110-v a-c source was not available. For 
these reasons, three of the instruments were 
remodeled so that the source operated from a 
6-v battery. This required an additional manual 
control for the source voltage. The sample tube 
also was lengthened, a 12-in. tube being used in 
place of the original 5-in. one. A converter was 
supplied for the amplifier to produce 110-v 
alternating current from the 6-v battery. The 
amplifier, however, could also be operated from 
a 110-v a-c line if desired. 


64 APPLICATIONS OF INFRARED 
DETECTORS AND ANALYZERS 5a 

The most immediate application of the infra¬ 
red detector was as a measuring device in con¬ 
nection with the design of ventilating systems 
for tanks. A problem arises because of gases 
which may be blown back into the tank when 
gun breeches are opened. The products of com¬ 
bustion of the powder contain important per¬ 
centages of carbon monoxide, and if this gas 
gets back into the tank, dangerous concentra¬ 
tions may develop. The gas-analysis apparatus 
described is particularly well adapted to supply 
the need for a device which will continuously 
indicate CO concentrations, as a running sample 
can be drawn from any desired region in the 
tank, and changes in CO concentration are 
quickly evident on the chart. Since this appli¬ 
cation requires CO measurements down to only 
about 0.01 per cent, which is readily detected, 
no temperature shielding is required, and meas¬ 
urements can be carried out in spite of gun 
concussion. 

Another application is to monitor the atmos¬ 
phere in submarines. In this case, the device is 
used to indicate or warn of excess C0 2 and is 


made responsive to relatively large concentra¬ 
tions (up to 5 per cent) by using a short 
sampling chamber. Tests' for CO can also be 
included. Such tests or routine studies can be 
made with present apparatus, and it could be 
incorporated into a relatively simple warning 
instrument to indicate the approach of danger¬ 
ous gas conditions automatically. 

There is also a definite need for a CO detector 
in connection with certain ventilating problems 
in airplanes. To the extent that such problems 
arise in ground tests or where laboratory-type 
installations can be made, the present apparatus 
is satisfactory. However, the more extreme con¬ 
ditions of sensitivity and stability required for 
a CO warning device (because dangerous con¬ 
centrations are quite low), together with the 
extreme temperature and pressure fluctuations, 
seem to preclude the possibility of developing 
the infrared detector for a simple CO warning 
device on airplanes in flight. 

Applications of infrared apparatus to the de¬ 
tection of war gases and tests of an apparatus 
for this purpose at the Edgewood Arsenal indi¬ 
cate that this detector is useful for measuring 
the so-called charcoal break in connection with 
the determination of the saturation of gas¬ 
mask absorbing materials. In such an analysis, 
air containing the gases to be detected is passed 
through the absorbing material of the gas mask 
and then into the analyzing chamber of the 
gas detector. The charcoal break occurs when 
the absorbing material becomes saturated for 
any constituent of the poisonous gas. When this 
occurs and the constituent begins to pass 
through the absorber, the fact is immediately 
indicated by the infrared detector. Since the 
detector is responsive to any of a wide range 
of hydrocarbon and other dipole molecules, some 
of which are almost certain to be present in 
any poisonous gas which might be considered, 
gas masks and other protective devices can be 
tested without the necessity of devising chemi¬ 
cal or other means for the detection of the 
specific gases involved. 


CONFIDENTIAL 



Chapter 7 

DETECTION OF PLASTIC PARTICLES 


71 INTRODUCTION AND SUMMARY 

T his report describes experimental work 
done in attempting to develop a method of 
detecting nonmetallic fragments which might 
become embedded in the human body during 
combat. The plastic used throughout the in¬ 
vestigation was Plexiglas. This is the trade 
name of a polymeric methyl methacrylate resin. 

The surgeons of the Ninth Air Force noted 
that 10 per cent of wounds sustained in combat 
by the crew members of medium bombers were 
due to fragments of Plexiglas from shattered 
windshields and windows. The surgical removal 
of these Plexiglas particles was slow and diffi¬ 
cult because the particles could be found only 
by probing the wound. X-ray visualization was 
impossible by standard X-ray techniques be¬ 
cause the radiographic opacity of Plexiglas is 
so similar to that of blood and muscle. 

The surgeon’s metal locator was useless for 
Plexiglas or other nonmetallic parts of airplane 
parts found in combat wounds. Its existence, 
however, inspired the hope that some similar 
locator could be developed for Plexiglas. 

A summary of reports from Air Force 
surgeons showed that injuries by Plexiglas 
were only 4 per cent of casualties in the Eighth 
Air Force and 10 and 1 per cent of casualties 
among bomber and fighter crews, respectively, 
in the Ninth Air Force. Only 3 to 10 per cent 
of the wounds by Plexiglas were deep, most 
of the wounds being superficial and presenting 
no particular difficulty in treatment. Those 
surgeons who desired a locator for Plexiglas 
asked that it be sensitive to particles 1 to 2 mm 
in size. 

Three methods of attack were studied ex¬ 
tensively. The first of these was the coating of 
Plexiglas with radioactive materials, the pres¬ 
ence of which might be detected with a Geiger- 
Mueller counter. The second was the develop¬ 
ment of a low-voltage X-ray technique; and the 
third was the opacification of Plexiglas. A 
brief inconclusive study was also made of the 
X-ray opacification of clothing. The results 


of these investigations are summarized below. 

A radioactive coating containing 10 micro¬ 
curies of Mn 52 per sq cm permits detection 
of fragments having more than 1 to 2 sq mm 
of surface. Among Plexiglas fragments 3 mm 
and smaller, produced by .50-caliber machine 
guns or 20-mm high explosive shells, less than 
one-third of the particles possess as much as 
1 sq mm of coated surface. In addition, a 
lacquer coating on Plexiglas produces optical 
surface irregularities. It does not appear as if 
surface coating is a practical solution for 
detection. 

The probability of detecting Plexiglas par¬ 
ticles less than 5 mm in diameter in tissue 
by means of the best low-voltage X-ray tech¬ 
nique developed is essentially zero. 

When Plexiglas was opacified to X-rays by 
a sufficient addition of organometallic or 
organic halogen compounds to be useful, it 
exhibited marked optical discoloration. 

Plexiglas particles, if initially sterile, cause 
no appreciable irritation of muscle. 

Fragments smaller than 3 mm have very 
little penetrating power, and can be stopped 
by a layer of light cloth. Those parts of the body 
which are not adequately protected by hair or 
regular clothing could be covered by a light 
garment or hood. Small particles will penetrate 
deeply into tissue only when the Plexiglas is 
adherent to a steel fragment. 


72 ANIMAL EXPERIMENTS 

Two sets of experiments with Plexiglas frag¬ 
ments were made with animals. The first set 
was to determine the amount of damage which 
might be done to dogs by flying Plexiglas frag¬ 
ments. For this purpose a cubical test chamber, 
4 ft on each side, was constructed. The Plexiglas 
panels, 2 ft by 2 ft by in., were mounted 
centrally with respect to one side. One shot 
was fired at each of three panels with experi¬ 
mental dogs in the chamber to study the pene¬ 
tration and subsequent location of plastic 


182 


CONFIDENTIAL 


DETECTION BY RADIOACTIVITY 


183 


fragments in animal tissue. High-explosive 20- 
mm shells were used. The fuzes were very 
sensitive, and the shells all exploded in contact 
with the Plexiglas panels. 

Three anesthetized dogs were used in these 
tests, all three being in the chamber at one 
time. They were laid on their sides, and a patch 
of the upper side of each was shaved clean to 
simulate human skin. 

A summary of the pathological studies in 
these three animals is significant because of 
its bearing on similar injuries which might 
occur to combat personnel under similar condi¬ 
tions. The wounds varied from tiny abrasions of 
less than 1 mm to shallow defects in the skin 
measuring 5 to 15 mm. There were few wider 
wounds and these were usually associated with 
particles of steel. No wounds extended more 
than 1.5 cm below the skin surface. Most 
involved the skin and subcutaneous tissue. Re¬ 
moval of Plexiglas from the subcutaneous tissue 
is more difficult than steel because of its marked 
adherence. This is probably caused by the multi¬ 
faceted character of the fragments. Very few 
wounds were present over the unshaved ex¬ 
posed surfaces of the dog’s body. Those present 
were quite large, deep, and contained Plexiglas 
and steel fused together, as well as large num¬ 
bers of hairs embedded in the base. Recognition 
of Plexiglas in the subcutaneous tissue is 
generally not difficult. The presence of a thick 
covering of hair is a good, but not complete, 
protection against Plexiglas injury. When such 
injuries do take place, they are associated with 
a considerable amount of crushing and inclu¬ 
sion of many well-embedded loose hairs. 

The second group of animal experiments 
were carried out to determine whether Plexi¬ 
glas particles which might be left in the body 
would have any harmful results. For this de¬ 
termination, five mice were anesthetized, and 
Plexiglas splinters were embedded in the thigh 
muscles, using sterile precautions. The mice 
were decapitated after 24, 68, and 120 hours 
and the thigh muscles studied. In every case, 
no edema or infection was observed, a healing 
scar appeared over the injection site, and the 
muscle appeared normal. The conclusion to be 
drawn from this set of experiments is that 
pieces of Plexiglas embedded in muscle produce 


very mild changes, if any. In support of these 
observations is the fairly extensive use of this 
type of plastic as surgical drainage tubes and 
bone cups in orthoplasty. 

The results of these animal experiments in¬ 
dicate that a light covering of cloth or heavy 
hair affords good protection from injury due 
to flying Plexiglas particles. Injury by the frag¬ 
ments, except where they are associated with 
pieces of steel, is never deep but usually results 
in abrasions to the skin and the immediately 
underlying subcutaneous tissues. Fragments of 
Plexiglas remaining in the body tissues cause 
only very mild inflammatory changes, or none 
at all. 


73 DETECTION BY RADIOACTIVITY 

Plexiglas shatters under the impact of 
antiaircraft shrapnel, explosive shells, and 
machine-gun bullets. The feasibility of coating 
the Plexiglas surface with a detectable coating 
was extensively studied. The usefulness of this 
type of detection depends on the size of particle 
to be detected. To meet the desired military 
requirements of detecting particles in the range 
from 1 to 2 mm or less, the results of these 
studies indicate that no surface coating would 
be practical. This conclusion is drawn from two 
series of tests, one with .50-caliber machine-gun 
bullets, and one with high-explosive 20-mm 
shells. The amount of surface carried by the 
Plexiglas fragments was studied by coating the 
inner surface with a radioactive coating, and 
testing the fragments for activity to obtain 
a result of the presence or absence of a detect¬ 
able amount of surface. For the .50-caliber 
machine-gun bullets, less than 20 per cent of 
the particles in the size range from 1 to 2 mm 
showed any detectable surface; for the 20-mm 
cannon shells, about 6 per cent could be de¬ 
tected. From these tests, it is apparent that 
coating the inner surface of all Plexiglas in 
aircraft will not solve the problem of locating 
plastic fragments in wounds. A large fraction 
of the smaller fragments, which are the most 
difficult to locate by standard surgical tech¬ 
niques, do not carry a detectable amount of 
inner surface. 


CONFIDENTIAL 



184 


DETECTION OF PLASTIC PARTICLES 


7 4 DETECTION BY LOW-VOLTAGE X-RAYS 

The most extensive experimental work was 
done in an attempt to evaluate the use of low- 
voltage X-rays in the detection of Plexiglas 
particles. To determine the conditions under 
which Plexiglas could be detected in the human 
body, use was made of a blood phantom. This 
consisted of a copper tank with a 1/32-in. bake- 
lite bottom. The tank was filled to various 
depths with whole blood from normal adults. 
Particles of shattered Plexiglas, which were 
screened in millimeter steps in known numbers, 
were sprinkled at random on the bottom of the 
tank. 

In the initial part of the investigation, use 
was made of a step wedge of Plexiglas with 
1/32-in. steps. During the part of the experi¬ 
ment in which the thin wedge was used in the 
phantom, the position of the wedge from the 
bottom of the tank was varied over wide limits. 
It was concluded that in every case contrast 
was very much more important than definition 
for the detection of the Plexiglas in blood. In 
the case of the shattered particles, the defini¬ 
tion was a maximum since the particles rested 
on the bottom of the tank. 

The procedure of setting up criteria for 
optimum detection under varying conditions 
was as follows: (1) screened particles of Plexi¬ 
glas having known sizes were radiographed 
through varying depths of blood at various 
voltages; (2) radiographs of the phantom were 
always taken of 100 particles; (3) the number 
of particles recognized as Plexiglas particles 
on the radiograph were counted by a highly 
trained roentgenologist. This determined the 
per cent recognition, which was used as a test 
of optimum technique, which field observation 
would parallel. 

It was found, as a result of this experiment, 
that the lower the voltage used, the better the 
contrast between the Plexiglas particles and 
the blood. One is limited by the time of ex¬ 
posure in the possible softness of the useful 
radiation. Since the exposure time increases 
with the square of the decrease in voltage, a 
certain rather critical voltage exists, below 
which the exposure time becomes unreasonably 
long. This critical voltage obviously depends on 


the thickness of material which the X-ray 
beam must penetrate. A family of curves is 
shown in Figure 1, in which exposures in milli- 
ampere-seconds are plotted as a function of 
peak voltage for a number of different blood 



Figure 1 . Exposures in milliampere-seconds as a 
function of peak voltage for a number of different 
blood thicknesses for a radiographic density con¬ 
venient for a standard diagnostic interpretation. 

thicknesses. These curves are all for the same 
radiographic density, chosen as convenient for 
standard diagnostic interpretation. 

One of the interesting findings of the investi¬ 
gation was the fact that an optimum film 
density exists. As the films become more dense, 
the per cent detection of the Plexiglas particles 
increases up to a broad maximum correspond¬ 
ing to a film density of about four, and there¬ 
after the per cent recognition decreases with 
increasing density. A plot of this result is 
shown in Figure 2, where a plot is made of the 
per cent recognition as a function of the ex¬ 
posure. 

The per cent recognition as a function of 
peak kilovolts for a number of different size 
Plexiglas particles in a fixed depth of blood 
at optimum density is shown in Figure 3. In 


CONFIDENTIAL 













DETECTION BY LOW-VOLTAGE X-RAYS 


185 


Figure 4 a plot is made of the per cent recogni¬ 
tion as a function of voltage for a fixed size of 
Plexiglas particle for a number of different 
blood levels. 

Experimental patch tests were carried out 
and showed no erythema reaction to an expo- 


DENSITY 



film density or milliampere-second exposure. 

sure of 720 ma-sec at 82 in. and 20.2 kv with 
a field size of 11 sq cm on the flexor surface 
of the forearm, using a fair-skinned individual. 
The number of roentgens delivered to the skin 
during this exposure was measured as 14. Since 



Figure 3. Per cent recognition as a function of 
peak voltage where a number of different size 
Plexiglas particles are in a fixed depth of blood 
at optimum density. 

this exposure was far greater than that used 
in the radiographic tests, it was concluded that 


the exposures were limited rather by the condi¬ 
tions of immobility than erythema. 

As a result of studying a number of different 
commercially available types of film, it was 
found that Eastman No-Screen film gave the 
optimum contrast. This film is slower than the 
usual film used in medical radiography (East¬ 
man Blue-Brand), but the gain in contrast was 
so marked in No-Screen over Blue-Brand, that 
the former is always to be recommended in this 
type of radiology. 

The fogging of the film due to backscattered 
radiation was remarkable in this low-voltage 
X-ray work. At 20 kv it was found experi¬ 
mentally that about half the density of a film 
backed with air, paper, or wood was due to 



Figure 4. Per cent recognition as a function of 
peak voltage for a fixed size of Plexiglas particle 
for a number of different blood levels. 


backscattered radiation. This backscattered 
radiation, since it blackens the film without 
giving a radiographic image of the Plexiglas, 
does no good whatever, and serves only to 
obscure the image of the Plexiglas particles. 
Radiographs in this investigation were always 
taken with the film intimately in contact with 
a lead backing % in. thick. 

An experimental tissue sample was prepared 
by removing one of the legs of the dogs which 
had been used in the experimental studies of 
Plexiglas fragments from explosive shells. 
Pieces of Plexiglas were embedded at different 
depths in the leg. The difficulty of detecting 
Plexiglas fragments in the inhomogeneous 
media of layers of muscle, fat, and other 
tissues was illustrated by radiographs of the 


CONFIDENTIAL 













186 


DETECTION OF PLASTIC PARTICLES 


dog’s leg. The Plexiglas is rendered very diffi¬ 
cult to detect when confused with soft tissue 
radiographic detail. The work with the ho¬ 
mogeneous phantom indicates the direction in 
which contrast can be improved, but the 
detection in homogeneous media will be much 
more probable than in the inhomogeneous 
medium of living tissue. 

A summary of the radiographic tests is as 
follows: contrast is the most important factor 
in detecting small particles of Plexiglas—the 
lower the voltage, the higher the contrast. For 
each given thickness of attenuating medium 
there is a minimum practical voltage deter¬ 
mined by the maximum practical exposure. 
There is an optimum density for maximum 
probability of detection, requiring a carefully 
controlled technique. Optimum results were 
obtained with the use of Eastman No-Screen 
film backed by %-in. sheet lead. The range of 
recommended exposures was found to be below 
erythema threshold. The probability of detect¬ 
ing Plexiglas particles in heterogeneous media 
is necessarily lower than in homogeneous 
media. The corresponding probabilities of de¬ 
tection of Plexiglas particles in the heteroge¬ 
neous radiographic media of human tissue will 
be lower. This effect is such that the probability 
of detecting particles below 5 mm in diameter 
in tissue is essentially zero. 


7 5 DETECTION BY X-RAY OPACIFICATION 

One of the methods available for rendering 
Plexiglas radiologically detectable is by incor¬ 
porating material of greater density into the 
methyl methacrylate sheeting. This possibility 
was investigated by preparing four samples 
differing in degree of transparency to X-rays. 
Information was not available as to what spe¬ 
cific materials had been added other than that 
they were organometallic and organic com¬ 
pounds. The one sample which was untreated 
showed no color. The other three samples 
showed distinct yellow coloration. Spectral 
photometric studies of these samples showed 
that absorption in the blue was very marked. 
In the case of the most heavily treated sample, 
transmission at the violet end of the spectrum 
was less by 20 per cent than in the case of the 
untreated sample, and less by 5 per cent in 
the red. No information was available on pos¬ 
sible changes in structural properties such as 
moldability of Plexiglas due to the opacifica¬ 
tion. 

Changing of the X-ray transparency of 
Plexiglas must be done with care since the 
density of Plexiglas for X-rays is originally less 
than that of blood. If the opacity of Plexiglas 
is not increased enough, there will be less 
contrast than there was originally. 


CONFIDENTIAL 



Chapter 8 

LOCATING UNEXPLODED BOMBS 

By F. L. Yost a 


INTRODUCTION 


T he specifications for a satisfactory bomb- 
locating instrument were as follows: 

1. The instrument must be capable of 
locating buried bombs under the following 
conditions. 


Weight of 
bomb 
100 lb 
500 lb 
1,000 lb 


Approximate Distance under 

dimensions surface 

8-in. diam, 30 in. long 15 ft 

15-in. diam, 48 in. long 20 ft 

19-in. diam, 54 in. long 25 ft 


These limits must be attainable, even though 
the bomb is in the vicinity of other buried 
objects, magnetic or otherwise, such as water 
pipes. 

2. The instrument must be capable of locat¬ 
ing in three dimensions a buried bomb to the 
following accuracy: horizontal direction, ±1 
ft; vertical direction, ±1 ft. 

8. If the instrument is not capable of locat¬ 
ing a buried bomb from the surface because 
of interference from buried objects, and if it 
becomes necessary to resort to the use of a 
bore-hole search coil, the design of the coil 
should be such that it can be used in a hole 
not over 6 in. in diameter (preferably smaller) 
and that it has a working range of at least 10 ft. 

4. It is desirable that not more than one hour 
should elapse from the time the instrument is 
placed in operation until the bomb is located. 

5. The equipment must be portable so that 
it can be operated at any site. 

6. The unit must operate without dependence 
on commercial power lines; the power required 
for operation must be small enough to be sup¬ 
plied by self-contained batteries or a portable 
gasoline-generator set. 

7. The equipment must be reasonably simple 
to operate and should be such that an average 
individual without technical training may 


a Technical Aide, Division 17, NDRC. 
b The control number for this project was OD-63. 


learn to use it effectively after brief instruc¬ 
tion. 

8. The equipment must not be extremely 
delicate. It should be at least as sturdy as 
ordinary radio equipment, so that average 
handling during transportation will not cause 
damage or put it out of adjustment. 

9. The design should be such that laboratory 
facilities are not necessary for making adjust¬ 
ments. Reference standards, if needed, should 
be self-contained. 

10. Any indicators should be visual rather 
than audible. 


8 2 SUMMARY OF DEVELOPMENT 

Work on this project was done by the Kan- 
nenstine Laboratories. A number of different 
methods of locating bombs were studied. 19 The 
most promising seemed to be magnetic locating 
methods. As a result, practically all the work 
on this project was in that direction. 

Preliminary investigations indicated that if 
the magnetic anomaly in the vertical component 
of the magnetic intensity resulting from the 
presence of an unexploded bomb was to be 
used to detect it, a magnetic gradiometer 
(which measures the gradient of the magnetic 
intensity) would be more suitable than a 
magnetometer (which measures the magnetic 
intensity). Two types of vertical-vertical gradi¬ 
ometer [VVG] were built for use at ground 
surface to determine the difference in vertical 
magnetic intensity at two different positions 
on a vertical line (i.e., to measure the vertical 
gradient of the vertical component of the 
magnetic intensity). Later a vertical-horizontal 
gradiometer [VHG] was developed and de¬ 
signed for use in bore holes to determine the 
difference in horizontal component of the mag¬ 
netic intensity at the different positions on a 
vertical line (i.e., to measure the vertical 
gradient of the horizontal component of the 


CONFIDENTIAL 


187 



188 


LOCATING UNEXPLODED BOMBS 


magnetic intensity). It was believed that this 
latter type of gradiometer would be more effec¬ 
tive in locating the position of a bomb. 

The VVG’s devised for use at ground surface 
did not fulfill requirements 1 and 2. In fact, 
further development would have been required 
to produce a satisfactory surface detector, even 
if tests with the experimental models had not 
indicated that the presence of extraneous mag¬ 
netic objects in the ground would make the 
usefulness of such a device quite limited. 6 ’ 8 
Accordingly, work was done on a VHG for use 
in a bore hole. 9 Although this instrument appar¬ 
ently met requirements 3, 5, 6, and 10, it 
probably would not have met requirement 4. It 
was merely a promising experimental model; 
hence no statement can be made as to whether 
requirements 7, 8, and 9 were met. 

The essentially inconclusive results of this 
project were in part caused by the fact that 
during work on it the problem of locating 
unexploded bombs decreased in importance. 
The press of work on more important projects 
resulted in termination of this contract with 
very little testing of the experimental models, 
and without following lines of investigation 
suggested by the tests which were run. For 
the purpose of recording the work done on the 
project should the problem become important 
again, this report discusses the bore-hole gradi¬ 
ometer and, in Section 8.4, the other less 
promising detection methods. 


8 3 DESCRIPTION AND TECHNICAL 
INFORMATION 

The VHG 9 for use in bore holes for locating 
unexploded bombs is the result of an extension 
of work done on two designs of VVG (for 
locating unexploded bombs by measurements 
made at ground surface), which will be dis¬ 
cussed later. Given any direction in the hori¬ 
zontal plane, the purpose of the VHG is to 
determine whether the magnetic fields in that 
direction differ at two different specified points 
along a vertical line, and, if so, to study the 
variation. As shown in Figure 1, the VHG con¬ 
sists of a pickup unit, which is to be lowered 
into a bore hole, and an indicating unit. 


The construction of the pickup unit is shown 
diagrammatically in Figure 2. The gradient- 
sensitive element consists of two narrow coils 
connected in series and supported, one above 
the other, by a comparatively long straight wire 
to the center of which is attached a mirror. 
This assembly is supported vertically by taut 



Figure 1. Pickup unit and indicating unit which 
make up the VHG. 


suspensions, which also serve as electric con¬ 
nections to the respective coils. The coils are 
so connected that the torque on one is opposite 
to that on the other when the system is in a 
uniform field and current is fed to the coils. 

The system can be adjusted to give zero 
oscillation of the mirror when it is fed by 
alternating current in a uniform magnetic field 
(i.e., one in which the field components at both 
coils and in their plane are the same, irrespec¬ 
tive of orientation of the coils). Maximum 
sensitivity in such adjustment is attained by 
use of an alternating current, the frequency 
of which is equal to that of the fundamental 
mode of vibration of the system. If this system 
is so driven in a field in which the horizontal 
components at the two coils and in their plane 
are different, the result is an oscillation of the 
mirror with an amplitude proportional to the 
driving current and the difference in the two 
horizontal components. In other words, the 
amplitude of oscillation is proportional to the 
product of the driving current and the vertical 
gradient of the horizontal field component in 
the plane of the coils. 


CONFIDENTIAL 




DESCRIPTION AND TECHNICAL INFORMATION 



.L. 

5 MM M 



4- DIA 
4 


15 MM 


-MIRROR 



40 TURNS NO. 44 
ENAMELED COPPER WIRE 


TOP VIEW 

DOUBLE COIL VIBRATOR AND MOUNTING 






PICKUP ASSEMBLY 


Figure 2. VHG pickup. 


189 


CONFIDENTIAL 



































































190 


LOCATING UNEXPLODED BOMBS 


The displacement of the mirror is converted 
into a voltage by means of a light source and 
photocell. This voltage is amplified and drives 
the vibrator (i.e., the mirror and coil system). 
The system is designed to constitute a feedback 
oscillator when the vibrator is oriented in a 
magnetic field having a sufficiently high 
gradient. The amplifier is provided with a 
delayed automatic gain control operating on 
the grid bias of one tube. A meter in the plate- 
supply circuit of this same tube serves as an 
indicator of gain. 

The indicating part of the gradiometer 
which remains at the surface contains all bat¬ 
teries and the whole amplifier, except a one- 
stage preamplifier which follows the photocell. 
It weighs 27 lb complete. The pickup weighs 
10.5 lb with the 25-ft connecting cable but 
without lowering rods. 

For the system to oscillate in a uniform 
external field, an internal gradient field, fixed 
with respect to the sensitive element, is pro¬ 
vided by a small permanent magnet. It should 
be weak enough to require the use of the 
highest usable amplifier gain for oscillation. 

When the system has been properly adjusted, 
the gain (i.e., the automatically controlled 
amplification applied to the voltage resulting 
from the oscillation of the system) required 
to maintain oscillation is independent of in¬ 
strument orientation in a uniform field; hence 
the meter reading, which indicates the gain, 
is independent of the orientation. If the instru¬ 
ment is in a magnetic field which has a detect¬ 
able vertical gradient of the horizontal com¬ 
ponent, the meter reading (i.e., the gain) varies 
with orientation in the field. The meter reading 
will be a minimum when the instrument is so 
oriented that the external and effective internal 
gradients are parallel; and it will be a maxi¬ 
mum when the external and internal gradients 
are antiparallel. Comparison of meter readings 
at different orientations serves to determine 
the magnitude and approximate direction of the 
external gradient. 

Although the scale is not actually linear, a 
rough determination of the sensitivity of the 
device indicates that it is about 10 gammas 
per ft per 0.01 ma. Tests were made outdoors 
above ground with a vertical pipe 79 in. long 


and 5.5 in. in diameter as the test object. Read¬ 
ings were with the pickup at about the height 
of the center of the test object and 7 ft west 
of it. The average difference in WE readings 
with and without the object was 0.12 ma; for 
NS readings, 0.02 ma (of the order of the 
average deviations). Computation of the direc¬ 
tion of the object by use of these data would 
result in an error of about 10 degrees (tan 1 
2/12). The worst pairing of single sets of data 
would give an error of about 27 degrees. On 
a subsequent test the direction calculated from 
the average data was in error about 15 degrees. 

The operation of the VHG is slower than that 
desired, as the time between steady readings 
is about one minute. In actual use a descent 
in the bore hole would be made at a fixed 
orientation, with readings being taken at inter¬ 
vals. If no significant anomaly appeared, an 
ascent would be made at an orientation 90 
degrees from the first. If this showed no sig¬ 
nificant anomaly, it would be concluded that 
the bore hole was not close enough to the bomb 
for the instrument to indicate its location. If 
an anomaly were found, readings at the depth 
of its maximum value would be taken in dif¬ 
ferent orientations to determine the direction 
of the gradient. The bomb depth would be 
approximately that of maximum anomaly, and 
the distance of the bomb from the bore hole 
could be estimated from the shape of the 
anomaly-depth curve. 

It can be said merely that a first model of 
a satisfactorily portable VHG suitable for 
locating buried iron-case bombs by measure¬ 
ments in bore holes of 4-in. diameter was 
developed. Its speed of operation does not meet 
requirements. It apparently has enough sensi¬ 
tivity to permit the location of a large bomb 
from a bore hole passing within 7 to 10 ft of it. 
However, it was not possible to make tests in 
actual bore holes or to attempt certain improve¬ 
ments of the instrument by elimination of 
difficulties discovered in preliminary testing 
before the date set for termination of the 
contract. The instrument is best described as 
a promising experimental model. If the problem 
of locating unexploded bombs again becomes 
important, it is conceivable that this instru¬ 
ment could be developed into a form suitable 


CONFIDENTIAL 



HISTORY 


191 


for field use, and that it would successfully 
accomplish its purpose. 


84 HISTORY 

The first attempt at magnetic location was 
directed toward the development of the VVG’s. 
The first model had several disadvantages, but 
was sufficiently promising to justify work on a 
second one in which the disadvantages of the 
first were eliminated as much as possible. 

The field-sensitive element in both gradi- 
ometers is a vibrating coil, or vibrator, similar 
in construction to a taut-suspension-oscillo- 
graph galvanometer. As shown in Figure 3, the 
coil is horizontal and centered between two 
coaxial Helmholtz coils whose axes are vertical. 



To measure the vertical gradient, two of these 
coils are used, one vertically above the other. 

In the first gradiometer, photocells were em¬ 
ployed to observe the vibrations of the coils. 
The lower of these was coupled to an amplifier 
driving the vibrators, and also to a current- 


control circuit automatically reducing the mag¬ 
nitude of the lower Helmholtz field almost to 
that of the vertical component of the earth’s 
field, which it opposes. The residual field is just 
strong enough to permit the lower system to 
function as an oscillator. The upper Helmholtz 
coils were connected in series with the lower, 
so that the upper field was approximately equal 
to the lower external field except for an addi¬ 
tive constant, and the total upper field was then 
the difference between upper and lower external 
fields except for an additive constant. The 
vibration of the upper moving coil served to 
measure this field; and knowledge of this field 
and the separation of the coils determined the 
gradient. This instrument had a precision of 
about 0.5 gamma per ft, but with further 
development higher precision could have been 
expected. 

Tests indicated moderate success in detecting 
buried magnetic objects simulating bombs, but 
revealed a number of disadvantages. A new 
VVG was designed to remove these difficulties. 

In the new system, each vibrator with its 
associated photocells and amplifier constitutes 
an oscillator driving the vibrator at its own 
resonant frequency. The essential feature of 
this second system is that the upper oscillator 
system controls not only its own Helmholtz-coil 
current, but also nearly all the lower Helmholtz- 
coil current. The lower oscillator system 
controls only the small fraction of lower 
Helmholtz-coil current required to adjust for 
the difference in vertical field between the 
upper and lower systems. This part of the 
lower Helmholtz-coil current may be made a 
small fraction of the lower control current, 
which is measured by the meter, by means of 
a simple resistance network. Thus, a relatively 
insensitive meter, in this case a 0-1 milli- 
ammeter, can be used to measure a change in 
lower Helmholtz-coil current which is a small 
fraction of the meter reading. It is a system 
superior to the first gradiometer. 

This and various other changes in design 
were more or less successful in removing diffi¬ 
culties. While the new gradiometer sometimes 
performed satisfactorily, there were frequent 
jumps in zero and, occasionally, extreme un¬ 
steadiness in indication, the causes of which 


CONFIDENTIAL 


































192 


LOCATING UNEXPLODED BOMBS 


were not determined. However, it was believed 
that an instrument of this type could be made 
to operate satisfactorily. 

In addition to the magnetic methods de¬ 
scribed, the following possible locating methods 
were considered and studied: (1) electrical 
methods—r-f and potential-drop ratio, (2) 
thermal methods, and (3) seismic methods. 

The r-f electrical methods similar to those 
used in anti-tank mine locators (e.g., a large 
exploring coil which is the inductance element 
of a vacuum-tube oscillator) did not appear to 
be promising, as they are inherently short- 
range methods and particularly affected by 
water pipes and other interfering material. 
Also, there seemed to be little value in another 
method in which a fixed transmitting antenna 
was used to produce a field in the vicinity of 
the conductor, and a portable directional re¬ 
ceiver was used to determine the direction of 
the minimum signal at various points. Large 
effects were produced by extended conductors 
such as water pipes, and no observable effects 
by compact conductors such as bombs. 

Conductivity measurements—the basis of 
the potential-drop-ratio method 3 —were not 


successful. Water pipes and other extraneous 
materials produced large anomalies, and small 
objects such as bombs produced no observable 
effects. 

Thermal methods were investigated theo¬ 
retically. 4 It was concluded that they would 
have very little application, if any. Observable 
temperature change would occur only after the 
lapse of a long time and only at short distances 
from the bomb. There were also practical 
objections, such as the difficulty of putting a 
thermometer in the earth without raising its 
temperature. There is a slight possibility of 
locating the path of the bomb by the tempera¬ 
ture rise, but this is of doubtful value. 

The use of seismic methods, utilizing waves 
which are refracted or reflected by the bomb, 
was not adequately tested by experiment, but 
it was believed to be impractical. Theoretically, 
the wavelength to be used should be short 
(probably about equal to the diameter of the 
bomb). Attenuation at such a wavelength is 
great, making it doubtful that sufficient energy 
could be sent to the bomb and reflected to the 
surface for observation, unless the source was 
extremely powerful. 


CONFIDENTIAL 



Chapter 9 


DEVELOPMENT OF AN ELECTROMAGNETIC MASS DETECTING 

SECURITY DEVICE 

By F. L. Yost a 


91 INTRODUCTION 

T his project resulted from a request of the 
U. S. Secret Service for a device which would 
indicate the approach of metallic objects, such 
as concealed weapons. 


92 REQUIREMENTS 

The apparatus was to detect metallic weap¬ 
ons, particularly pistols, concealed on a person 
passing through a constricted passageway, 
e.g., a doorway 4 ft wide by 7 ft high. The 
detection was to be made without the knowledge 
of the person under surveillance. Some of the 
existing devices for such a purpose had harmful 
effects on watches; others were difficult to 
conceal; and most of them failed to provide 
satisfactory discrimination between weapons 
and common metal objects of small size. It was 
desired that these difficulties be corrected and 
that the final apparatus be such that it could 
be operated by personnel with a minimum of 
training. 

The requirements were not made more spe¬ 
cific because it was not known how much could 
be done in the way of distinguishing between 
a concealed metallic weapon and a legitimate 
object such as a cigarette lighter or a tobacco 
can. Therefore, the project was originally 
exploratory in nature; after it became evident 
that some degree of success could be achieved 
in the problem, work was devoted to improving 
the most promising method. 

9 3 SUMMARY OF DEVELOPMENT 

An attempt was first made to develop an in¬ 
strument which would detect metallic objects 
by their magnetic effects. 1 The difficulties of 

a Technical Aide, Division 17, NDRC. 


discrimination in this method seemed insur¬ 
mountable. It was, therefore, discarded in favor 
of a method in which metallic objects were to 
be detected by their disturbance of the flux 
density of an alternating electromagnetic field 
through which they passed. 2 ' 3 

It was found that the proper choice of field 
frequency assisted in discrimination of the type 
of metallic object detected. In addition, a 
thorough study was made of other factors 
which influence discrimination, and attention 
was given to improving sensitivity and to re¬ 
ducing effects from extraneous alternating 
fields (particularly 60 c). 

In the final development, the apparatus indi¬ 
cated the sizes of guns quite satisfactorily. 
There was some overlap between the smallest 
guns and pocket tobacco cans. Cases for glasses 
were found to be about as effective as .25-caliber 
automatics. It was also found that officers’ caps 
with steel bands have a strong effect. However, 
no other articles were found to give misleading 
indications; pocket knives and keys barely reg¬ 
istered. In spite of the minor discrimination 
difficulty still remaining, the U. S. Secret Serv¬ 
ice was satisfied with the development. 

As finally installed for operation, the unit 
proved satisfactory except for disturbance suf¬ 
fered from moving metal parts which had not 
been removed, as planned, from the doorway 
in which it was placed, and a high level of inter¬ 
ference from air-conditioning apparatus located 
near-by. This interference seriously hampers 
the use of the apparatus, but the field coil can 
be enlarged and its field increased sufficiently 
to override the interference. 


94 DESCRIPTION AND TECHNICAL 
INFORMATION 

In general, a device which will indicate the 
presence of an iron or steel object that passes 


CONFIDENTIAL 


193 



194 DEVELOPMENT OF AN ELECTROMAGNETIC MASS DETECTING SECURITY DEVICE 


through an alternating electromagnetic field 
must depend for its operation on some disturb¬ 
ance of that field. This may be accomplished in 
several ways, such as (1) by a disturbance of 
the flux density of the field, (2) by a phase shift 
between two points in the field, (3) by a change 
in the mutual inductance between two or more 
coils in the field, or (4) by a change in either 
the inductance or the Q of a coil in the field. 
Tests were made on all the methods mentioned. 
It was found that the one giving the most posi¬ 
tive indication with the least amount of equip¬ 
ment required, and generally best suited to the 
purpose described, was the flux-density-dis¬ 
turbance method, of which the mutual induc¬ 
tance method is a special case of practical value. 

Figures 1, 2, and 3 illustrate the general idea 
of the method 2 (although the final number and 
arrangement of coils were not exactly as shown 
in Figures 1 and 2). As shown in Figure 1, a 
field coil which sets up an alternating electro¬ 
magnetic field is so arranged that its axis passes 
through the mid-point of a small pickup coil, 
the axes of the two coils being perpendicular 
to each other. 

Figure 2 shows the coils of Figure 1, viewed 
from directly above. This figure shows two pos¬ 
sible sets of magnetic-flux lines set up by the 


✓ 



Figure 1. General arrangement of field coil and 
pickup coil. 


field coil. The dotted lines represent a series of 
undistorted flux lines, while the dashed ones 
represent a distorted series. Complementary 
lines are marked 1 and 1', 2 and 2', etc. The 
normal, undistorted flux lines are more or less 
perfectly symmetrical about the field-coil axis. 
Thus, for each line passing to the left of the 


pickup coil, a complementary line passes to the 
right. Accordingly, the pickup coil has zero 
voltage induced in it from the field coil if it is 
set in the position shown. The dashed lines 


FIELD COIL 


- $:/. - 


FLUX- DISTURBING 
OBJECT 


7#iA, " 


OBJECT ~1 

i|! \ v. 

1 -0/ / /• ill: \- \ • 

''/■ / /• HI; V s 
7 —■ y- / /■ M \. V 

.. / / /.'9o«j.:\- pic 


/ /•* 90 ° 1 T‘\ ' PICKUP — 

/.-1 £ | \ v. 

/,•' .V 

.• \ •. V- 

.•/ .• \ •. x \ •*.. 
/ \ \ 


V 


* 5 ' 


Figure 2. Effect of a flux-disturbing object on 
the electromagnetic field between field coil and 
pickup coil. 


represent the magnetic-flux lines when dis¬ 
torted by the presence of a flux-distorting ob¬ 
ject. They correspond roughly to those repre¬ 
senting the undistorted field pattern, but they 
are shifted in position. Because of the field dis¬ 
tortion, the pickup coil is no longer balanced 
with respect to the field coil, and some voltage 
is induced in it. The amount of induced voltage 
depends on several factors, such as (1) the 
amount of distortion of the flux pattern which 
would be affected by the mass, shape, electric 
conductivity, magnetic permeability of the dis¬ 
torting object, and the position where introduced 
into the field, (2) the strength of the electro¬ 
magnetic field at the pickup coil, (3) the core 
material, (4) the length of the core, and (5) 
the number of turns on the pickup coil. 

The distorted field resulting from the pres¬ 
ence of the flux-distorting object may be de- 


CONFIDENTIAL 







DESCRIPTION AND TECHNICAL INFORMATION 


195 


scribed in two ways: it may be said that the 
original field is warped by the object; or it may 
be said that the exciting field is still in its 
original form, but that a new or secondary field 
is added to it by each disturbing object. By 
either method the same result is achieved, be¬ 
cause the primary field plus the secondary field 
is equivalent to the warped field. The secondary 
components of fields for various objects may 
have different directions at a given point be¬ 
cause the disturbing objects are in various posi¬ 
tions; and the phases of the components differ 
with the electric properties of the objects. Cop¬ 
per and iron, for example, cause effects nearly 
180 degrees apart in phase, and therefore prac¬ 
tically opposite in effect. 

For an undistorted field, Figure 3 shows how 
the induced voltage varies when the pickup coil 



DEGREES ROTATION OF PICKUP COIL IN PLANE 
PERPENDICULAR TO PLANE OF FIELD COIL 

Figure 3. Induced voltage as a function of de¬ 
grees of rotation of pickup coil in plane perpen¬ 
dicular to plane of field coil. 

is rotated to make its axis approach coincidence 
with that of the field coil. Theoretically, the 
induced voltage reaches a maximum when the 
axis of the pickup coil coincides with that of the 
field coil; actually, the amplifier which would be 
associated with the pickup coil would overload 
long before any such degree of rotation would 
be reached, so that Figure 3 illustrates the 
effect of only a small amount of rotation. 

The maximum detecting effect is obtained if 
the system operates at the null point, which is 
at the cusp of the curve. In practice the pickup 
coil responds not only to the direct field-coil 
component and the secondary component of the 
object being detected, but also to other un¬ 
changing components which cannot be cancelled 
against one another because of difference in 


phase, direction, and strength. Accordingly, as 
explained later, it is necessary to introduce a 
buckout voltage in the pickup coil to obtain 
the null point electrically. 

The induced voltage in the pickup coil does 
not depend on the rate of motion of the metal¬ 
lic object, but rather on its position relative to 
the two coils. When a flux-distorting object is 
passed between the field coil and the pickup coil 
in a direction parallel to the core of the pickup 
coil, there will be an induced voltage as the 
object approaches the coils. The voltage will 
reach a maximum in one direction when the 
object is in such a position relative to the two 
coils during its approach as to produce the 
greatest amount of flux distortion. As the ob¬ 
ject is passed between the two coils, the induced 
voltage drops, passing through the zero value, 
and then rises to a maximum in the opposite 
direction when the object is in such a position 
that it produces the greatest flux distortion on 
the other side of the pickup coil. As the object 
continues its motion, the induced voltage re¬ 
turns to its normal operating value. 

In the final arrangement, two pickup coils 
were used, neither on the axis of the field coil. 
The arrangement of the coils and the associated 
apparatus is as shown in Figure 4. The com- 



Figure 4. Arrangement of coils and associated 
apparatus in a door frame. 

plete problem resolved itself into a number of 
subproblems: the determination of a suitable 
field frequency and the designing of a satisfac¬ 
tory oscillator and associated field coil which 


CONFIDENTIAL 
















196 DEVELOPMENT OF AN ELECTROMAGNETIC MASS DETECTING SECURITY DEVICE 


would be moderate in size but would set up a 
fairly strong field; the designing of pickup 
coils, together with suitable arrangements for 
obtaining null points; and the designing of an 
amplifier which would give sufficient sensitivity 
with the existing field, and which at the same 
time would not be affected by extraneous or 
transient fields. 

Preliminary tests 2 in the frequency range 
from 1,000 to 10,000 c showed that an object 
made of nonferrous material, such as copper, 
aluminum, or brass, produced distortion of the 
electromagnetic field ranging from slight 
amounts at 1,000 c to approximately 50 per 
cent of that produced by a small tobacco tin at 
10,000 c. As it was undesirable to have non- 
ferrous articles produce indications, a frequency 
below 1,000 c was indicated. A frequency as 
low as 60 c was undesirable, because work with 
that frequency had shown that there were seri¬ 
ous interference effects. Accordingly, a field fre¬ 
quency of 500 c was decided upon, that par¬ 
ticular value being chosen because it was 
thought at the time that a commercial alter¬ 
nator might be used. (It was later decided to 
build an oscillator-amplifier.) 

Work on an oscillator-amplifier passed 
through a series of phases and trials 3 until an 
oscillator-amplifier and associated field coil 
were developed which were sufficiently power¬ 
ful to set up the strongest electromagnetic field 
required. The final circuit design 5 is shown in 
the transmitter unit [TU] in Figure 5. It has 
a power rectifier, using a 5U4-G tube in a con¬ 
ventional circuit. In addition to high voltage 
for the 6L6 power amplifiers, a stabilized lower 
voltage is provided for the 6SN7 oscillator. 
Two VR-150-30 gas tubes control the voltage 
roughly. The 6J5 tube which follows is a surge 
suppressor which irons out minor jumps of 
voltage that may occur in the VR-150-30 gas- 
tube regulators. 

The 6SN7 oscillator drives push-pull 6L6 
tubes, which deliver their output to the field 
coil. The field coil is a push-pull tuned coil con¬ 
nected directly in the plate circuit and designed 
to suit the tube impedance. Because of tuning, 
the circulating current in the field coil is much 
heavier than the plate current of the tubes. 
The field strength measured at the center of 


the field coil is 21.8 gauss; and the power dis¬ 
sipated in it is about 23 watts. 

It was originally planned to use Mu-metal 
cores in the pickup coils, but some concern was 
felt as to the relative ease with which such 
cores could be saturated by interfering fields. 
Tests showed there was not much probability 
of such a difficulty arising; but, to avoid the 
possibility of trouble, high-impedance audio¬ 
transformer windings were used as air-core 
coils. A pickup coil is fastened on each side of 
the door, 2 to 3 in. above the threshold and con¬ 
cealed by the door frame. Some allowance is 
made for longitudinal, horizontal, and vertical 
adjustment and rotation of the coil. 

As mentioned above, it proved impracticable 
to obtain null points for the pickup coils solely 
by their adjustment relative to the field coil. 
It was, therefore, decided to adjust the coils 
for a moderately good balance and to complete 
the null by bucking the residual voltage out 
electrically with a component of controllable 
phase and amplitude. The circuit which fur¬ 
nishes the buckout voltage is shown in the TU 
in Figure 5. The phase is adjusted by means 
of two phase-shifting networks in cascade. Each 
network provides a range greater than 90 de¬ 
grees, giving a total of over 180 degrees. The 
amplitude range is controlled by a single po¬ 
tentiometer of 0.2 megohm, wire wound. 

There is another voltage injected into the 
receiver unit [RU] (see Figure 5) from the 
TU, the sensitizing or control frequency, 
which will be discussed later. This voltage is 
adjusted roughly in phase by means of the 
“control-frequency-phaser” tap switch. It is 
merely set on the position that gives the largest 
deflection when a sample gun is carried through 
the doorway. 

The development of a receiving amplifier was 
beset with considerable difficulty in eliminating 
interference. At first a selective amplifier built 
on the heterodyne principle was tried, with 
two resistance-coupled stages to discriminate 
against 60-c pickup. There was too much inter¬ 
ference from 60 c and harmonics produced by 
small motors and transformers near the pickup 
coils. Filters to remove 60-c interference and 
low harmonics were tried, but higher harmonics 
were found to be of sufficient importance to 


CONFIDENTIAL 



TRANSMITTER UNIT 


DESCRIPTION AND TECHNICAL INFORMATION 


197 



CONFIDENTIAL 

































































































































































































































198 DEVELOPMENT OF AN ELECTROMAGNETIC MASS DETECTING SECURITY DEVICE 


make this method impossible. Finally the RU 
illustrated in Figure 5 was developed. 

In the RU the signal path starts at the lower 
left corner of the diagram and follows clock¬ 
wise around it. The input to the first 6J5 tube 
includes the signal from the pickup coils and 
the buckout voltage which enters through a 
high-pass filter. The first three 6J5 tubes are 
used as a resistance-capacitance tuned, inverse 
feedback amplifier that is highly selective to 
the 500-c signal. (The amplifier bandwidth is 
about 5 c.) This is followed by the 6SN7 ampli¬ 
fier stage and the 6H6 diode discriminator. The 
discriminator is sensitized or controlled by fre¬ 
quency from the power oscillator so that it is 
highly selective to the desired phase and fre¬ 
quency. (The discriminator bandwidth is about 
% c.) The use of extreme selectivity does not 
demand equivalent stability of the 500-c oscilla¬ 
tor and discriminator, since the discriminator 
response inherently follows any drift of the 
oscillator frequency. 

The resulting d-c output fluctuations pass 
through the 4,000-henry choke into a 0.25-^f 
condenser, to eliminate interfering kicks of very 
short duration. The following condenser of 0.5 
fxf and resistor of 2 megohms eliminate any 
slow drifts due to temperature changes or other 
causes. 

The second 6SN7 stage drives the recorder. 
It is connected bridgewise with two resistors 
and a VR-90-30 gas tube. The recorder current 
depends on the difference between the fixed 
gas-tube voltage and the variable plate voltage 
of the 6SN7. 

The 6J5 tube in the lower right corner of the 
diagram serves as a sort of logarithmic volt¬ 
meter. Its grid is excited by the signal in the 
amplifier, and its plate-current variations are 
indicated by the 0-200 microammeter. The 
meter reads about 170 or 180 for zero a-c input, 
which occurs when the buckout controls are per¬ 
fectly adjusted. The meter reading drops for 
increased input voltage, but the drop is less and 
less rapid as voltage increases. This permits the 
same meter to be used for preliminary rough 
adjustments and the final balance. 

The indications are received as deflections on 
a pen-type recorder. 5 The apparatus is adjusted 
so that the pen draws a line down the center of 


the paper chart. If a gun is brought through 
the doorway, the pen first swings in one direc¬ 
tion, reaching a peak deflection when the gun 
is about 2 ft from the plane of the doorway. 
The deflection then reverses, swings across the 
center line as the gun passes the plane of the 
doorway, and reaches a reverse peak when the 
gun is about 2 ft beyond the doorway. There is 
some lag in pen response, but the operator may 
easily make allowance for it. A piece of non¬ 
magnetic metal, if highly conductive like copper 
or silver, will show up with deflections opposite 
to those expected for iron, and much smaller 
for a given amount of metal. 

In addition to the recorder, a sound alarm 
(not shown in circuit diagram) is provided. 5 
The sound is a tone which is interrupted or con¬ 
tinuous, depending on the strength of the indi¬ 
cation. A small speaker in the control unit and 
two extension speakers carry this sound to the 
desired locations. Each speaker has a volume 
control. The speakers are turned on by three 
relays which may be set to trip at three desired 
values of the indication. The first relay, which 
trips on a small signal, puts the sound on with 
rapid interruptions. The second relay reduces 
the interruptions to a slow rate; and the third 
relay, which trips on the strongest deflection, 
makes the tone continuous. The relays stay 
closed until manually reset. 


95 HISTORY 

The method first tried 1 was to detect weapons 
by their magnetic effect. An attempt was made 
to develop an electrical apparatus which would 
be sensitive to the presence of very small varia¬ 
ble magnetic fields but insensitive to the pres¬ 
ence of the very much stronger fixed component 
of the earth’s field at the pickup units. 

Pickup coils designed to detect very small 
changes superimposed on the earth’s field were 
to be arranged as shown in Figure 6. Coils 
marked A-A, B-B, etc., were to be connected in 
series opposition so that the effects of distant 
magnetic objects would cancel out, whereas the 
effect of a magnetic object nearby could not 
do so. 

A general idea of the operating principle of 


CONFIDENTIAL 



HISTORY 


199 


each detecting unit (i.e., each pair of coils in 
series opposition) can be obtained from the 
block diagram of Figure 7. Each pickup coil 
consists of a primary coil and a secondary coil. 


DOORWAY 



PANEL 

Figure 6. Schematic arrangement for magnetic 
detection method. 

Pulses from the oscillator saturate the primary 
circuit of the pickup coils. The voltage developed 
in the secondary of the pickup coil depends 
critically on the component of magnetic field 


appears as a biasing voltage at the grid of 
the pentode amplifier. This voltage is the differ¬ 
ence of the two voltages developed in the sec¬ 
ondary coils, since the secondaries are in series 
opposition. The actual voltage developed at the 
diode is much less than the total voltage induced 
in each secondary coil. Any change in the ex¬ 
ternal magnetic field around the pickup coils 
will cause a proportional change in the sec¬ 
ondary-circuit voltage, thus producing more or 
less rectified voltage at the diode detector. This 
voltage is filtered and passed directly to the 
pentode amplifier. 

Changes in plate voltage at the pentode am¬ 
plifier are passed on to the phase-inverter stage, 
which divides the voltage changes into two 
channels. One channel produces an increase in 
voltage for an increase in voltage at the pentode 
amplifier plate; the other channel produces a 
decrease in voltage for an increase in voltage 
at the pentode amplifier plate. This method was 
chosen so that the control relays actuating the 
indicating apparatus could be of the simple 
nonpolarizing type. For any change in the 
magnetic field greater than a fixed minimum 



imposed parallel to the axis of the coil. The 
fundamental idea of the arrangement is that 
the presence of a metallic object will affect the 
voltage developed in the secondary circuit of 
the pickup coil. Energy from the secondary cir¬ 
cuit is rectified at the diode detector and 


value at the pickup coils, one or the other of the 
relays will operate. Changes in plate voltage at 
the output of the phase-inverter stage are am¬ 
plified by the buffer stage and passed on to the 
final power amplifier stage, which is capable of 
developing a sufficiently large current change in 


CONFIDENTIAL 














































200 DEVELOPMENT OF AN ELECTROMAGNETIC MASS DETECTING SECURITY DEVICE 


its plate circuit to actuate the control relays. 
These relays operate the two indicating lamps, 
one for each channel. The current meter is in 
the common plate-supply circuit to the final 
amplifier stage, and shows an increase in cur¬ 
rent for a change of the magnetic field in either 
direction at the pickup coils. 

Considerable work was expended on this 
method, but it offered so many difficulties in 
obtaining a sufficiently steady oscillator and in 
discrimination that it was ultimately aban¬ 
doned. It was found that a gun could have a 
very weak magnetization of its own, just strong 
enough to be neutralized in certain positions 


by the earth’s field. This occasionally resulted 
in zero indication. On the other hand, small 
objects such as mechanical pencils were too 
often found to be magnetized so highly that 
their effects were similar to that of an average 
gun. 

Variations of the method were also consid¬ 
ered, such as magnetization of all objects before 
attempted detection, demagnetization before at¬ 
tempted detection, and a system of magnetiza¬ 
tion and detection immediately followed by 
demagnetization and detection. All such mag¬ 
netic methods were finally discarded in favor 
of the electromagnetic-field method. 


CONFIDENTIAL 



Chapter 10 

SURGEON’S METAL LOCATOR 


10.1 INTRODUCTION AND SUMMARY 

T his project was undertaken with the hope 
of improving upon the locators already 
available. These locators are of the probe type 
and depend for their operation upon the change 
in coupling between two coils wound together 
upon the probe when a small ferrous object is 
brought near the probe. The coils are connected 
in a bridge circuit, and the degree of unbalance 
is indicated on a sensitive meter. The disad¬ 
vantages of this type of instrument are the in¬ 
convenience of using a meter as an indicator, 
the lack of sensitivity to nonferrous objects, and 
the inability to indicate depth. 

The locator, as finally developed, uses an audi¬ 
ble signal as an indicator. When a metallic ob¬ 
ject is brought near the probe, the pitch of the 
audible signal is changed. The instrument de¬ 
pends for its operation upon the change in fre¬ 
quency of an r-f oscillator by eddy currents set 
up in a metallic object brought near the probe. 
This locator does not suffer from the first two 
difficulties mentioned above, but, like the other 
probe-type locators, cannot indicate depth. A 
caliper-type locator was developed which could 
be used to indicate depth. Since it depended 
upon the magnetic properties of the object it 
was insensitive to nonferrous objects. It also 
proved unstable and hard to balance, and was 
unsatisfactory except in the hands of a skilled 
operator. It was therefore decided to use a 
probe-type locator as a supplement to X-ray 
investigations. In use, the approximate position 
of the object is found by X-ray, the approximate 
surgical approach is made, and the locator used 
to guide further incision. 


10 2 EXPERIMENTAL LOCATORS 1 - 2 

Much of the development work done on loca¬ 
tors was concerned with the caliper type in the 
hope that a workable instrument could be de¬ 
veloped which would make possible the localiza¬ 
tion of the object in three dimensions, and thus 


remove the need for X-rays. The instrument 
developed used two primary coils as one side of 
an inductive bridge. The two secondary coils, 
which are on the two sides of the caliper (shown 
in Figure 1), are connected in parallel opposi¬ 
tion. This arrangement was found empirically 



Figure 1 . Caliper form of surgeon’s metal locator. 


to give the most stable and most sensitive loca¬ 
tor. The instrument was first tried using 60-c 
current, but it was found that the 60-c pickup 
from the room rendered the locator very un¬ 
stable. Accordingly, 30-c current was used in 
the coils, and a low-pass filter was put in the 
amplifier to remove the 60-c pickup. This large 
filter and the motor-generator set (which sup¬ 
plied the 30-c current) made the instrument 
quite bulky. The output of the bridge is ampli¬ 
fied, rectified, and applied to the grid of a 
relaxation oscillator which produces an audible 
signal. The introduction of a ferrous object be¬ 
tween the coils produces an unbalance of the 
bridge and a change in the audible signal, with 
the change being greatest when the object is 
directly between the coils. This locator proved 
capable of locating an object within a region 
about V 2 in. in diameter. However, it was bulky, 
used quite a bit of power, and had a tendency 
to be unstable except in the hands of a skilled 
operator. 

A similar arrangement was tried using an 


CONFIDENTIAL 


201 





202 


SURGEON’S METAL LOCATOR 


r-f current instead of the 1-f current. In this 
locator the two primary coils were connected in 
series and served as the tank coil of a 456-kc 
crystal-controlled oscillator. The particular fre¬ 
quency was chosen because of the ready avail¬ 
ability of components. The two pickup coils 
were again connected in parallel opposition. 
This locator proved to be moderately sensitive 
and of equal sensitivity for nonferrous and 
ferrous materials. However, it proved quite un¬ 
stable and had one insurmountable difficulty, 
namely, the absorption of power by the patient’s 
body. 

Other methods which were tried, but which 
were not so successful and were not developed 
further, include: a method in which the effi¬ 
ciency of a tuned transformer is changed by 
the proximity of a metallic object; a method 
in which the inductance is used in a phase-shift¬ 
ing circuit, a change in the plate current of a 
thyratron tube being caused by a change in the 
inductance; and an entirely different scheme in 
which a beam of X-rays passed through the 
body and was received on a fluorescent screen, 
the resulting light being measured by a photo¬ 
cell. 


10.3 THE FINAL LOCATOR 

The locator, as finally developed and manu¬ 
factured, is shown in Figure 2. The exploring 
point of the probe consists of an oscillator coil, 
whose associated tube, choke, and condensers 
are contained in the handle of the probe. The 
frequency of operation of the system is ap¬ 
proximately 5.5 me. The control box contains 
another oscillator, whose frequency can be 
varied by the tuning control. Signals from the 
two oscillators are fed to the grids of a mixer 
tube, and the beat note produced is amplified 
and fed to the loud-speaker. 

The locator is sensitive to metals by reason of 
their conducting properties. When a piece of 
conducting metal is brought into the r-f field 


of the probe oscillator, eddy currents are pro¬ 
duced, and the frequency is increased. If the 
tuning control is set so that the frequency of 
the control-box oscillator is below that of the 



Figure 2. Final production model of surgeon’s 
metal locator. 


probe oscillator, the approach of the probe to a 
metallic object will produce a rise in frequency 
of the audible signal. 

In the case of iron and steel, the effect of eddy 
currents in increasing the frequency is balanced 
to some extent by the increased inductance and 
decreased frequency caused by the magnetic 
permeability. An almost complete balance be¬ 
tween the two effects is obtained on a piece 
about 1 mm in diameter and 1 cm long. A 
smaller magnetic needle causes a lowering of 
pitch. For pieces of iron larger than 3 mm in 
diameter, the effect is not noticeable, and these 
pieces can be found as readily as nonferrous 
metallic objects. 

The locator is meant to be used in conjunction 
with X-rays, as described in the introduction, 
although it is frequently possible to locate an 
object from the surface. The large probe will 
indicate the presence of a .45-caliber lead bullet 
at a distance of IV 2 in. or a .22-caliber short 
lead bullet at % in. 


CONFIDENTIAL 





GLOSSARY 


a-RAY. A doubly ionized helium atom. 

Anti-personnel Mine. A low-pressure-functioning 
mine (perhaps 5 lb) with a small explosive charge 
(e.g., 200 g TNT) capable of being actuated by a 
man’s weight. 

Anti-tank Mine. A mine containing an explosive 
charge sufficient to disable a tracked vehicle (a charge 
of, say, 10 lb of TNT) and usually requiring several 
hundred pounds force to actuate. Some anti-tank 
mines, such as the Japanese Type 3 Flower Pot, are 
both anti-personnel and anti-tank since they are 
actuated by a few pounds’ weight. 

Gamma. 1 gamma is 10 —5 oersted. 

7 -Ray. A high-frequency electromagnetic wave pulse 
emitted by radioactive atoms. 


( 

Geiger-Mueller Counter. A gas-filled tube by which 
7 -rays and other rays are detected by means of a 
cascade ionization process. 

JANET Board. Joint Army and Navy Experimental 
and Testing Board. 

Metallic Mine. A mine containing sufficient metal 
parts (perhaps only a spring) to be detectable by a 
mutual inductance-type mine detector. 

Nonmetallic Mine. A land mine fabricated entirely 
of nonmetallic components. 

RU. Receiver unit. 

TU. Transmitter unit. 

VHG. Vertical-horizontal gradiometer. 

VVG. Vertical-vertical gradiometer. 


CONFIDENTIAL 


203 




BIBLIOGRAPHY 


Numbers such as Div. 17-111.11-MI indicate that the document listed has been microfilmed and that its 
title appears in the microfilm index printed in a separate volume. For access to the index volume and to 
the microfilm, consult the Army or Navy agency listed on the reverse of the half-title page. 


Chapter 1 

1. Portable Iron Detector, F. Wenner, R. J. Duffin, 
and J. L. Dalke, OSRD 4659, OEMsr-151, Depart¬ 
ment of Terrestrial Magnetism, Carnegie Institu¬ 
tion of Washington, Jan. 15, 1945. 

Div. 17-111.11-MI 

2. Mine Detector, SW-7, for Both Metallic and Non- 
Metallic Land Mines, H. G. Doll, Electro-Mechan¬ 
ical Research, Inc., July 19, 1944. 

Div. 17-111.31-MI 

3. Non-Metallic Mine Detector, H. G. Doll, C. B. 

Aiken, and 0. H. Huston, OSRD 4614, OEMsr- 
1063, Electro-Mechanical Research, Inc., Dec. 22, 
1944. Div. 17-111.21-M2 

4. Detection of Small Foreign Bodies Imbedded in 
the Top Soil by Low Frequency Alternating Cur¬ 
rent, Haakon M. Evjen and W. V. Mills, OSRD 
5677, OEMsr-1470, Elflex Company, Sept. 28, 1945. 

Div. 17-111.2-M10 

5. Location of Antitank Mines — I, L. F. Curtis, Re¬ 

port 1191-W, Hazeltine Service Corporation, Mar. 
11, 1941. Div. 17-112-MI 

6. Location of Antitank Mines — II, Fisher M-Scope, 

L. F. Curtis, Report 1192-W, Hazeltine Service 
Corporation, Mar. 12, 1941. Div. 17-112-MI 

7. Location of Antitank Mines — III, Enrico Terrom- 
eter—Model RF-6X, L. F. Curtis, Report 1193-W, 
Hazeltine Service Corporation, Mar. 13, 1941. 

Div. 17-112-MI 

8. Location of Antitank Mines — IV, Progress Report, 

L. F. Curtis, Report 1194-W, Hazeltine Service 
Corporation, Mar. 14, 1941. Div. 17-112-MI 

9. Location of Antitank Mines — V, Coil Arrangement 

for Detecting Metal Bodies in Induction Fields, 
L. F. Curtis, Report 1198-W, Hazeltine Service 
Corporation, Apr. 29, 1941. Div. 17-112-M2 

10. Location of Antitank Mines — VI, Differential Elec- 
tromagnetometer. Theory and Design Considera¬ 
tions, Rudolf C. Hergenrother, Report 1207-W, 
Hazeltine Service Corporation, May 28, 1941. 

Div. 17-112-M3 

11. Location of Antitank Mines — VII, Progress Report, 
June 1, 19^1, L. F. Curtis, Report 1208-W, Hazel¬ 
tine Service Corporation, May 28, 1941. 

Div. 17-112-M4 

12. Location of Antitank Mines — VIII, The Effect of 

the Ground on a Nearby Coil, H. A. Wheeler, Re¬ 
port 1218-W, Hazeltine Service Corporation, June 
19, 1941. Div. 17-112-M5 

13. Location of Antitank Mines — IX, Locator Using 
Amplified Frequency Variations, L. F. Curtis, Re¬ 


port 1223-W, Hazeltine Service Corporation, July 
28, 1941. Div. 17-112-M6 

14. Location of Antitank Mines — X, The Relative Ad¬ 

vantages of Certain Features, H. A. Wheeler, Re¬ 
port 1229-W, Hazeltine Service Corporation, 
Aug. 5, 1941. Div. 17-112-M7 

15. Location of Antitank Mines — XI, The Properties 

of Spherical Coils and Objects, H. A. Wheeler, Re¬ 
port 1230-W, Hazeltine Service Corporation, 
Aug. 8, 1941. Div. 17-112-M8 

16. Location of Antitank Mines — XII, Differential 
Electromagnetometer — Developmental Model, 
Rudolf C. Hergenrother, Report 1236-W, Hazeltine 
Service Corporation, Aug. 20, 1941. Div. 17-112-M9 

17. Location of Antitank Mines — XIII, Scale Model for 

Determining the Effects of Conducting Bodies and 
Conducting Media in Induction Fields, D. E. 
Blanchard, Report 1235-W, Hazeltine Service Cor¬ 
poration, Aug. 20, 1941. Div. 17-112-M10 

18. Location of Antitank Mines — XIV, Relative Re¬ 

sponse To Nearby Objects and Ground, H. A. 
Wheeler, Hazeltine Service Corporation, Aug. 
21, 1941. Div. 17-112-M11 

19. Location of Antitank Mines — XV, Theory of the 

Scale Model, H. A. Wheeler, Hazeltine Service Cor¬ 
poration, Aug. 22, 1941. Div. 17-112-M12 

20. Location of Antitank Mines — XVI, Tests of Locator 

Using Amplified Frequency Variation, T. C. Hana, 
Report 1239-W, Hazeltine Service Corporation, 
Aug. 28, 1941. Div. 17-112-M13 

21. Location of Antitank Mines — XVII, Interpretation 

of Tests on Scale Model, H. A. Wheeler, Report 
1241-W, Hazeltine Service Corporation, Aug. 
28, 1941. Div. 17-112-M14 

22. Location of Antitank Mines — XVIII, Summary [to] 
August 30, 194.1, L. F. Curtis, Report 1242-W, 
Hazeltine Service Corporation, Aug. 29, 1941. 

Div. 17-112-M15 

23. Location of Antitank Mines — XIX, Hazeltine 

Model 3 Locator, L. F. Curtis, Hazeltine Service 
Corporation, Oct. 4, 1941. Div. 17-112-M16 

24. Location of Antitank Mines, Summary, L. F. 

Curtis, Hazeltine Service Corporation, Nov. 26, 
1941. Div. 17-112-M17 

25. Location of Antitank Mines, Technical Features of 

Model 3 Locator, J. J. Okren, Hazeltine Service 
Corporation, Nov. 28, 1941. Div. 17-112-M17 

26. Location of Antitank Mines, Properties of a Con¬ 

ductive Object in an Alternating Magnetic Field, 
H. A. Wheeler, Hazeltine Service Corporation, 
Dec. 8, 1941. Div. 17-112-M17 

27. Location of Antitank Mines, Summary, L. F. 

Curtis, Hazeltine Service Corporation, Dec. 29, 
1941. Div. 17-112-M17 


CONFIDENTIAL 


205 


206 


BIBLIOGRAPHY 


28. Location of Antitank Mines, Description of Audio 

Detector, L. F. Curtis, Hazeltine Service Corpora¬ 
tion, Feb. 6, 1942. Div. 17-112-M17 

29. Location of Antitank Mines, Locator for Non- 
Metallic Mines, D. E. Blanchard, Hazeltine Service 
Corporation, Apr. 29, 1942. Div. 17-112-M17 

30. Location of Antitank Mines, Phase Discrimination 
Audio Detector, L. F. Curtis and J. E. Everett, 
Hazeltine Service Corporation, May 7, 1942. 

Div. 17-112-M17 

31. Marking of Friendly Mines, Lanes, and Booby- 

Traps by Radio Activity (Mamie) and Tests of 
Radioactivity-Methods for Locating Unmarked 
Enemy Mines (Dinah), Robley D. Evans, Sanborn 
C. Brown, and John W. Irvine, Jr., OSRD 3679, 
OEMsr-1156, Massachusetts Institute of Tech¬ 
nology, Apr. 10, 1944. Div. 17-111.2-M6 

32. Investigations of Microwave Means for the De¬ 

tection of Land Mines, George B. Hoadley and 
Charles A. Hachemeister, OSRD 5718, OEMsr- 
1374, Polytechnic Institute of Brooklyn, Oct. 31, 
1945. Div. 17-111.2-M12 

33. Detection of Land Mines, A. H. Kettler, R. E. 

Swain, H. P. Raab, J. B. Gehmann, H. J. Woll, 
G. W. Demuth, and W. J. Morlock, OSRD 4017, 
OEMsr-1061, Radio Corporation of America, Mar. 
15, 1944. Div. 17-111.2-M5 

34. Detection of Land Mines, W. J. Morlock, E. S. 
Lundie, H. J. Woll, J. B. Gehmann, and R. E. 
Swain, OSRD 4605, OEMsr-1061, Radio Corpora¬ 
tion of America, Nov. 15, 1944. Div. 17-111.2-M9 

35. Detection of Land Mines by Radio Frequency 
Methods, A. A. Barco, F. C. Gow, E. F. Coleman, 
and B. F. Miller, OSRD 4998, OEMsr-1061, Radio 
Corporation of America, Mar. 30, 1945. 

Div. 17-111.3-MI 

36. Detection of Land Mines, E. S. Lundie, H. J. Woll, 

J. B. Gehmann, and R. E. Swain, OSRD 5721, 
OEMsr-1061, Radio Corporation of America, Oct. 
31, 1945. Div. 17-111.2-M11 

37. Investigation of a Method of Non-Magnetic Mine 

Detection Based on the Well-Logger, Joseph Razek 
and H. W. Ashton, OSRD 4237, OEMsr-998, Oct. 

15, 1944. Div. 17-111.2-M7 

38. Mine Detection Method Proposed by Virgil Brit¬ 

tain, Joseph Razek and H. W. Ashton, OSRD 4238, 
OEMsr-998, Oct. 15, 1944. Div. 17-111.2-M8 

39. Seismic Method for Locating Non-Metallic Land 

Mines, S. A. Scherbatskoy, OEMsr-958, Apr. 30, 
1943. Div. 17-111.2-MI 

40. Memorandum on Nuclear Method for Locating Non- 
Metallic Land Mines, S. A. Scherbatskoy, Research 
Project 2503, May 10, 1943. 

41. Memorandum on Seismic and Nuclear Methods for 
Locating Non-Metallic Mines, S. A. Scherbatskoy, 
June 10, 1943. 

42. Location of Land Mines through Electromagnetic 
Methods, C. H. Fay, OSRD 5719, OEMsr-1463, 
Shell Oil Company, Inc., Oct. 31, 1945. 

Div. 17-111.3-M2 


43. The Location of Non-Metallic Land Mines by the 

Use of High-Frequency Oscillations, L. G. Ellis, 
R. L. Henson, J. W. Millington, and A. C. Winter¬ 
halter, OSRD 3089, OEMsr-998, Sun Oil Company, 
Feb. 15, 1944. Div. 17-111.2-M3 

44. The Location of Non-Metallic Land Mines by the 

Use of Vibrational Devices [such as] the Resonant 
Method [and] the Diffraction Method, L. G. Ellis, 
R. L. Henson, J. W. Millington, and A. C. Winter¬ 
halter, OSRD 3090, OEMsr-998, Sun Oil Company, 
Feb. 15, 1944. Div. 17-111.2-M4 

45. Electro-Mechanical Type of Detector for the De¬ 

tection of Non-Metallic Land Mines, E. A. Eck- 
hardt, OSRD 3396, OEMsr-998 and OEMsr-1061, 
Sun Oil Company and Radio Corporation of Amer¬ 
ica, Mar. 15, 1944. Div. 17-111.21-MI 

46. Detection of Non-Metallic Mines by Radioactivity 
Methods, Gerhard Herzog, OSRD 3158, OEMsr- 
1114, The Texas Company, Dec. 30, 1943. 

Div. 17-111.2-M2 

47. Observations on Land Mine Problem in North 
Africa and Italy, H. K. Stephenson, a memorandum 
to Chief, Section 17.1, NDRC, May 18, 1944. 

Div. 17-100-MI 

48. “Protection of Radium Dial Workers and Radi¬ 
ologists from Injury by Radium,” R. D. Evans, 
Journal of Industrial Hygiene and Toxicology, 
25:253-269, September 1943. 

49. “Radium Painting—Hazards and Precautions,” K. 
Morse and M. H. Kronenberg, Industrial Medicine, 
December 1943. 


Chapter 2 

1. Two Types of Mechanical Mine Exploders, L. J. 
Savage and W. Prager, AMP Memo 69.1, AMG- 
Brown University, Oct. 9, 1943. Div. 17-121.1-MI 

2. Model Study of the Rotaflail, B. R. Teare, Jr., 

OSRD 3023, OEMsr-1234, Carnegie Institute of 
Technology, Dec. 27, 1943. Div. 17-121.1-M2 

3. Model Study of the Rotaflail (Second Report) W. 
Caywood, OSRD 4197, OEMsr-3765, Carnegie In¬ 
stitute of Technology, Nov. 1, 1944. 

Div. 17-121.1-M3 

4. Model Study of the Rotaflail (Final Report), W. 
Caywood, OSRD 5165, OEMsr-1234, Carnegie In¬ 
stitute of Technology, June 5, 1945. 

Div. 17-121.1-M4 

5. A Study of the Dynamic Characteristics of Anti¬ 
tank Mines and the Development of Indicator 
Mines, Thomas Bardeen and A. P. Palmer, OSRD 
3884, OEMsr-266, Gulf Research and Develop¬ 
ment Company, June 1, 1944. Div. 17-122.1-MI 

6. The Effect of Shock Impulse on Antitank Mines 

(Part I and Part II), T. Bardeen, OSRD 6078, 
Gulf Research and Development Company, Oct. 
15, 1945. Div. 17-122.1-M3 and M4 


CONFIDENTIAL 



BIBLIOGRAPHY 


207 


7. The Detonation of Universal Indicator by Blast 

from Point Charges, J. L. Brenner, OSRD 5271b, 
Report AES-llb, Princeton University, June 25, 
1945. Div. 2-100-MI 

8. Effect of Blast on Indicator Mines , E. Bright 

Wilson, Jr., A. H. Taub, C. W. Lampson, and 
Thomas Bardeen, OSRD 4276, OEMsr-266, Prince¬ 
ton University, Stanolind Oil and Gas Company, 
and Gulf Research and Development Company, 
Sept. 28, 1944. Div. 17-122.1-M2 

9. The Effect of Line Charges on Universal Indicator 

Mine, J. L. Brenner and A. H. Taub, Report AES- 
10c, OSRD 5144c, Princeton University, May 25, 
1945. Div. 2-100-Ml 

10. The Detonation of TMi-U3 by Blast, J. L. Brenner, 

OSRD 5179, OEMsr-260, Princeton University, 
June, 1945. Div. 2-111.12-M4 

11. Resume of the Theory of Plane Shock and 
Adiabatic Waves with Applications to the Theory 
of the Shock Tube, C. W. Lampson, Report PTM- 
108, Apr. 27, 1945. 

12. Firing Device for the Projected Line Charge, 
Thomas Bardeen, OSRD 6077, OEMsr-266, Gulf 
Research and Development Company, Oct. 15, 1945. 

Div. 17-121.3-MI 

13. Developments Related to the Amphibious Snake, 
Thomas Bardeen, OSRD 5673, OEMsr-266, Gulf 
Research and Development Company, Oct. 15, 1945. 

Div. 17-121.2-MI 

14. Equipment for the Passage of Enemy Minefields 
and Passage of Beach and Underwater Obstacles, 
Engineer Board. See also Progress Reports of 
JANET Board. 


Chapter 3 

1. Magnetometers and Magnetic Gradiometers, 

Vaughn L. Agy, J. L. Dalke, R. J. Duffin, B. 
Tuckerman, and F. Wenner, OSRD 3339, OEMsr- 
151, Carnegie Institution of Washington, Feb. 28, 
1945. Div. 17-131.1-MI 

2. Magnetic Firing Device, F. Wenner and A. G. 
McNish, OSRD 4705, OEMsr-151, Carnegie In¬ 
stitution of Washington, Feb. 28, 1945. 

Div. 17-123-MI 

3. Magnetic Fields of Tanks and Other Vehicles, A. G. 

McNish, Bryant Tuckerman, Vaughn L. Agy, and 
J. M. Barry, OSRD 4734, OEMsr-151, Service 
Project OD-46, Carnegie Institution of Wash¬ 
ington, Feb. 28, 1945. Div. 17-122.2-MI 

3a. Ibid., pp. 20-25. 

3b. Ibid., Figs. 25-83, pp. 28-47. 

3c. Ibid., pp. 25-27. 

4. Magnetic Fields of Vehicles, Firing Device, and 
Detectors, Final Report on Contract OEMsr-151 
and Supplements, J. A. Fleming and A. G. McNish, 
OSRD 4809, OEMsr-151, Mar. 15, 1945. 

Div. 17-122.2-M2 


5. Magnetic Detector for Firing Antitank Mines, 
Gary Muffly and L. E. Smith, OSRD 5671, OEMsr- 
266, Gulf Research and Development Company, 
Oct. 1, 1945. Div. 17-123-M2 


Chapter 4 

1. Control System for Submarine Mines, Alfred B. 
Miller and Willard P. Place, Progress Report 82, 
OEMsr-328, Research Project D3-51P2, Union 
Switch and Signal Company, July 29, 1941. 

Div. 17-132.11-MI 

2. Control System for Submarine Mines, L. O. Gron- 
dahl, Progress Report 184, OEMsr-328, Union 
Switch and Signal Company, Feb. 11, 1942. 

Div. 17-132.11-M2 

3. Control System for Submarine Mines, L. O. Gron- 
dahl, Willard P. Place, and Alfred B. Miller, 
Progress Report 333, OEMsr-328, Union Switch 
and Signal Company, Nov. 17, 1942. 

Div. 17-132.11-M3 

4. Control System for Submarine Mines, Final Re¬ 
port Covering Period from January 15, 19^2 to 
June 30, 19J+3, OEMsr-328, Union Switch and 
Signal Company, July 23, 1943. Div. 17-132.11-M4 
4a. Ibid., pp. 4-12. 

5. Studies on Investigations Related to the Develop¬ 
ment of Detecting Devices for Magnetic Mines, 
Gary Muffly, Progress Report 146, OEMsr-95, Gulf 
Research and Development Company, Sept. 1, 1941. 

Div. 17-131-MI 

6. Studies on Investigations Related to the Develop¬ 
ment of Detecting Devices for Magnetic Mines, 
Gary Muffly, Progress Report 168, OEMsr-95, Gulf 
Research and Development Company, Nov. 1, 1941. 

Div. 17-131-MI 

7. Studies on Investigations Related to the Develop¬ 
ment of Detecting Devices for Magnetic Mines, 
Gary Muffly, Progress Report 182, OEMsr-95, Gulf 
Research and Development Company, Jan. 2, 1942. 

Div. 17-131-MI 

8. Studies on Investigations Related to the Develop¬ 
ment of Detecting Devices for Magnetic Mines, 
Gary Muffly, Progress Report 240, OEMsr-95, Gulf 
Research and Development Company, Mar. 2, 1942. 

Div. 17-131-MI 

9. Studies on Investigations Related to the Develop¬ 
ment of Detecting Devices for Magnetic Mines, 
Gary Muffly, Progress Report 276, OEMsr-95, Gulf 
Research and Development Company, May 1, 1942. 

Div. 17-131-MI 

10. Studies on Investigations Related to the Develop¬ 
ment of Detecting Devices for Magnetic Mines, 
Gary Muffly, Progress Report 277, OEMsr-95, Gulf 
Research and Development Company, July 1, 1942. 

Div. 17-131-MI 

11. Studies on Investigations Related to the Develop¬ 
ment of Detecting Devices for Magnetic Mines , 


CONFIDENTIAL 




208 


BIBLIOGRAPHY 


Gary Muffly, Progress Report 346, OEMsr-95, Gulf 
Research and Development Company, Sept. 1, 
1942, Nov. 1, 1942. Div. 17-131-MI 

12. Detecting and Firing Device for Magnetic-Influence 

Ground Mines, Gary Muffly, OSRD 1463, OEMsr- 
266, Gulf Research and Development Company, 
Jan. 1, 1943. Div. 17-132.111-MI 

13. Detecting and Firing Device for Magnetic-Influence 

Ground Mines , Gary Muffly, OSRD 1464, OEMsr- 
266, Gulf Research and Development Company, 
Mar. 1, 1943. Div. 17-132.111-M2 

14. Magnetic-Influence Ground Mine Detector and Re¬ 
lated Indicating and Firing Equipment , Gary 
Muffly, OSRD 1557, OEMsr-266, Gulf Research 
and Development Company, Sept. 15, 1943. 

Div. 17-132.11-M5 

14a. Ibid., p. 31. 

15. Magnetic-Influence Underwater Ground Mines, 
OSRD 1999, OEMsr-266 and OEMsr-328, Gulf Re¬ 
search and Development Company and Union 
Switch and Signal Company, Dec. 10, 1943. 

Div. 17-132.11-M6 

16. Design of Acoustical Controlled Ground Mine, 

OEMsr-47, Massachusetts Institute of Technology, 
Oct. 29, 1941. Div. 17-132.12-MI 

17. Design of Acoustical-Controiled Ground Mines, 

Massachusetts Institute of Technology, Dec. 27, 
1941. Div. 17-132.12-M2 

18. A Design of an Acoustical Control in the New 

Army Controlled Ground Mines, Progress Report 
179, Massachusetts Institute of Technology, Feb. 
6, 1942. Div. 17-132.12-M3 

19. A Design of an Acoustical Control in the New 

Army Controlled Ground Mines, Progress Report 
202, Massachusetts Institute of Technology, Mar. 
30, 1942. Div. 17-132.12-M3 

20. Recent Tests on a Model TH-2 Microphone, Sup¬ 
plementary Progress Report 219, OEMsr-47, Mas¬ 
sachusetts Institute of Technology, May 16, 1942. 

Div. 17-132.12-M4 

21. Acoustic Echo Device and Other Acoustic Detectors 

for Controlled Submarine Mines, Progress Report 
306, OEMsr-295, Massachusetts Institute of Tech¬ 
nology, Sept. 30, 1942. Div. 17-132.12-M5 

22. A Listening-Type Acoustical Firing Unit for Con¬ 
trolled Mine Warfare (Type T-5E18), Cyril M. 
Harris, OSRD 1629, OEMsr-295, Massachusetts In¬ 
stitute of Technology, June 24, 1943. 

Div. 17-132.12-M6 

23. Type T-5E18 — Production Model, Cyril M. Harris, 
Supplement to OSRD 1629, OEMsr-295, Massa¬ 
chusetts Institute of Technology, Sept. 1, 1943. 

Div. 17-132.12-M6 

23a. Ibid., p. 3. 

Chapter 5 

1. History of Project 17.3-3. 

2. Errors in Sound Ranging, R. B. Lindsay, OSRD 


3807, OEMsr-734, Duke University, Apr. 22, 1944. 

Div. 17-434.2-M2 

3. New Analytical Methods of Computing Sound 
Source Locations, R. B. Lindsay, OSRD 3808, 
OEMsr-734, Duke University, Apr. 22, 1944. 

Div. 17-434.1-MI 

4. Nomographic Method of Computing Sound Source 
Locations, R. B. Lindsay, OSRD 3809, OEMsr-734, 
Duke University, Apr. 22, 1944. 

Div. 17-434.1-M2 

5. Acoustic Wave Front Corrugations in the Atmos¬ 
phere, R. B. Lindsay, OSRD 3810, OEMsr-734, 
Duke University, Apr. 22, 1944. Div. 17-433-MI 

6. Studies of Microphone Characteristics for Gun 
Ranging, H. C. Silent, OSRD 4075, OEMsr-734, 
Duke University, Oct. 19, 1945. Div. 17-434.411-Ml 

7. Seismic Artillery Ranging, H. C. Silent, H. C. 
Rothenberg, and John C. Stick, Jr., OSRD 4353, 
OEMsr-734, Duke University, Oct. 10, 1944. 

Div. 17-434.412-MI 

8. Dodar, A Short-Base Sound Ranging System, M. J. 

Burger, OSRD 5538, OEMsr-734, Duke University, 
July 21, 1945. Div. 17-434.31-MI 

9. Dodar, Development of Improved Model Signal 

Corps No. AN/PNS-1, T. G. Barnes and R. W. 
Collins, OSRD 5539, OEMsr-734, Duke University, 
Oct. 10, 1945. Div. 17-434.31-M2 

10. Fluctuations of Atmospheric Sound Transmission, 

R. L. Wegel, OSRD 5540, OEMsr-734, Duke Uni¬ 
versity, Nov. 30, 1945. Div. 17-432-MI 

10a. Ibid., Fig. 6-7. 

10b. Ibid., Fig. 8-15. 

11. Sound Ranging Nomograms and Associated Equip¬ 
ment, E. B. Nichols and F. E. White, OSRD 5541, 
OEMsr-734, Duke University, Dec. 15, 1945. 

Div. 17-434.33-M2 

12. Ballistic-Burst Method of Sound Ranging, R. H. 

Frick, Jr., OSRD 5542, OEMsr-734, Duke Uni¬ 
versity, Sept. 29, 1945. Div. 17-434.35-M2 

13. Binaural Listening Systems, F. S. Claassen and 

W. C. Ranes, Jr., OSRD 5543, OEMsr-734, Duke 
University, Nov. 7, 1945. Div. 17-434.32-MI 

14. Highly Portable Sound Ranging Microphone, John 

C. Stick, Jr., OSRD 5544, OEMsr-734, Duke Uni¬ 
versity, Nov. 6, 1945. Div. 17-434.31-M3 

15. Artillery Plotting Grids, E. B. Nichols, OSRD 5545, 
OEMsr-734, Duke University, Dec. 17, 1945. 

Div. 17-434.331-MI 

16. Dry Paper Sound Ranging Recorder, F. S. Claassen, 

OSRD 5546, OEMsr-734, Duke University, Nov. 
28, 1945. Div. 17-434.34-MI 

17. Design of Watertight Equipment Cases, E. B. 

Nichols, OSRD 6265, OEMsr-734, Duke Uni¬ 
versity, Dec. 15, 1945. Div. 17-434.31-M4 

18. Analysis of Gun Ranging Records, Report IC-1, 

Duke University, Feb. 12, 1945. 

Div. 17-434.41-MI 

19. Discussion of Army Ground Forces Reports, 
C-Misc-30, European Theater of Operations, and 


CONFIDENTIAL 



BIBLIOGRAPHY 


209 


C-Misc-29, European Theater of Operations, F. E. 
White, Report IC-2, Duke University, Mar. 2, 1945. 

Div. 17-434.2-M3 

20. Description and Use of Nomographic Charts in 

Sound Ranging, F. E. White, Report IC-3, Duke 
University, June 7, 1945. Div. 17-434.33-MI 

21. The T-l Microphone Developed by the Division of 

Physical War Research, Duke University, Durham, 
N. C., John C. Stick, Jr., Report IC-4, Duke Uni¬ 
versity, May 22, 1945. Div. 17-434.35-MI 

22. Possible Use of Doppler Effect in Sound Ranging, 

H. C. Silent and R. B. Lindsay, Report IC-5, Duke 
University, Oct. 26, 1945. Div. 17-434.5-MI 

23. Proposed Method of Sound Ranging, Eliminating 
Meteorological Connections, R. H. Frick, Jr., Re¬ 
port IC-6, Duke University, Dec. 6, 1945. 

Div. 17-434.35-M3 

24. Preliminary Investigation of Sphinx Project, R. W. 

Collins and I. Rudnick, Report IC-7, Duke Uni¬ 
versity, Nov. 3, 1945. Div. 17-434.5-M2 

25. Errors in Sound Ranging by the Use of Three 
Collinear Microphones, R. B. Lindsay, OSRD 1135, 
OEMsr-667, Brown University, Nov. 25, 1942. 

Div. 17-434.2-MI 

26. War Department Field Manual, FM 6-120, May 
1945. 

27. Sound Ranging for Artillery, Vol. 2, SCL Engi¬ 
neering, Report No. 753, Jan. 1, 1942. 

28. Instruction Book for Sound Ranging Set GR-3-C, 
OCSigO. 

29. Analysis of Sounds from Field Artillery and Ma¬ 

chine Guns, Vol. I, J. B. Kelley, OSRD 4594, 
OEMsr-498, Bell Telephone Laboratories, Inc., 
Jan. 15, 1945. Div. 17-422-M3 

30. Triangular Array Method, SCL Engineering Re¬ 
ports S-6 and 753. 

31. Goodwin Method, EBD Reports 16, 17, 18, and 19. 

32. A Discussion of the Accuracy of Sound Ranging, 
Army Operational Research Group, Report 121. 

33. Seismic Ranging Research, SCL Engineering Re¬ 
port S-2. 

34. A New Type of Selective Circuit, H. H. Scott, 
Proc. I.R.E., 26, 226-235 (1938). See also, Variable 
Frequency Oscillators, W. C. Shepherd and R. 0. 
Wise, Proc. I.R.E., 28, 256-268 (1943). 

35. Electric Filter, H. W. Augustadt, U. S. Patent No. 
2,106,785. 

36. Bridged-T and Parallel-T Null Circuits, W. E. 
Tuttle, Abs., Proc. I.R.E., 26, 675 (1938). 

37. Ultra-High-Frequency Techniques, J. G. Brainerd, 
G. Koehler, H. J. Reich, and L. F. Woodruff, D. 
Van Nostrand Co., N. Y., 1942, pp. 177-179. 

38. Sommerfeld, Ann. Phus. 28, 665 (1909), and 81, 
1635 (1926). 


Chapter 6 

1. Gas Detection and Analysis, A. H. Pfund, Progress 
Report 84, Johns Hopkins University, July 31, 

1941. Div. 17-210-MI 

2. Gas Analysis Contract, William G. Fastie and 

A. H. Pfund, Johns Hopkins University, January, 

1942. Div. 17-210-M2 

3. Gas Analysis Contract, William G. Fastie and 

A. H. Pfund, Johns Hopkins University, August, 

1942. Div. 17-210-M2 

4. Gas Analysis Contract, William G. Fastie and 

A. H. Pfund, Johns Hopkins University, April, 

1943. Div. 17-210-M2 

5. Selective Infrared Gas Analyzers, William G. 
Fastie and C. Wilbur Peters, OSRD 5674, OEMsr- 
178, Johns Hopkins University, Oct. 31, 1945. 

Div. 17-210-M5 

5a. Ibid., pp. 42-49. 

6. Selective Gas Analyzers, J. R. Stewart, OSRD 

5004, OEMsr-1035, Leeds and Northrup Company, 
May 31, 1945. Div. 17-210-M4 

7. Infrared Gas Detector and Gas Analyzer, OSRD 

1642, OEMsr-178 and OEMsr-1035, Johns Hopkins 
University, and Leeds and Northrup Company, 
Jan. 1, 1944. Div. 17-210-M3 


Chapter 7 

1. Study of Methods for the Detection of Plastic 
Particles in Human Bodies, Robley D. Evans, 
Sanborn C. Brown, and John W. Irvine, Jr., OSRD 
5678, OEMsr-1489, Massachusetts Institute of 
Technology, Sept. 29, 1945. Div. 17-221-M4 


Chapter 8 

1. Location of Unexploded Aerial Bombs — I, F. M. 

Floyd, Report RP-1, Kannenstine Laboratories, 
Nov. 7, 1942. Div. 17-222-MI 

2. Location of Unexploded Aerial Bombs — II, F. M. 

Floyd, Report RP-2, Kannenstine Laboratories, 
Mar. 1, 1943. Div. 17-222-MI 

3. Investigation of the Possibility of Locating Buried 
Bombs by Electrical Surveys, C. H. Fay, Report 
RP-3, Kannenstine Laboratories, Mar. 3, 1943. 

Div. 17-222-M2 

4. A Theoretical Investigation of the Problem of Un¬ 

exploded Bomb Detection by Thermal Means, C. H. 
Fay, Report RP-4, Kannenstine Laboratories, Mar. 
17, 1943. Div. 17-222-M3 

5. Problem of Locating Unexploded Bombs by Vari¬ 

ous Geophysical Means, Final Report, F. M. 
Kannenstine, Kannenstine Laboratories, Mar. 22, 
1943. Div. 17-222-M4 


CONFIDENTIAL 



210 


BIBLIOGRAPHY 


6. A Magnetic Gradiometer for Unexploded Bomb 
Location, C. H. Fay, Report RP-5, Kannenstine 
Laboratories, Mar. 31, 1943. Div. 17-222.1-MI 

7. Location of Unexploded Aerial Bombs — III, F. M. 

Floyd, Report RP-6, Kannenstine Laboratories, 
June 16, 1943. Div. 17-222-MI 

8. A Second Vertical-Vertical Magnetic Gradiometer 
for Unexploded Bomb Location , Report RP-7, 
Kannenstine Laboratories, Jan. 24, 1944. 

Div. 17-222.1-M2 

9. A Vertical-Horizontal Gradiometer for Unexploded 

Bomb Location by Measurements in Boreholes, Re¬ 
port RP-8, OEMsr-749, Kannenstine Laboratories, 
Jan. 24, 1944. Div. 17-222.1-M3 


Chapter 9 

1. Development of a Magnetic Mass Detecting Secu¬ 

rity Device, Gary Muffly and L. E. Smith, OEMsr- 
266, Gulf Research and Development Company, 
Sept. 1-Nov. 1, 1942. Div. 17-223-MI 

2. Development of an Electromagnetic Mass Detecting 

Security Device, Gary Muffly and L. E. Smith, 
OEMsr-266, Gulf Research and Development Com¬ 
pany, Jan. 1-Mar. 1, 1943. Div. 17-223-MI 


3. Development of an Electromagnetic Mass Detect¬ 

ing Security Device, Gary Muffly and L. E. Smith, 
OEMsr-266, Gulf Research and Development Com¬ 
pany, May 1-July 1, 1943. Div. 17-223-MI 

4. Development of an Electromagnetic Mass Detecting 

Security Device, Gary Muffly and L. E. Smith, 
OEMsr-266, Gulf Research and Development Com¬ 
pany, Sept. 1, 1943. Div. 17-223-Ml 

5. Electromagnetic Mass Detecting Security Device, 

Gary Muffly and L. E. Smith, OSRD 5330, OEMsr- 
266, Gulf Research and Development Company, 
July 21, 1945. Div. 17-223-M2 


Chapter 10 

. 1. The Construction of an Improved Metallic Object 
Locator, W. E. Gilson, Progress Report 312, Uni¬ 
versity of Wisconsin, Nov. 6, 1942. 

Div. 17-221-MI 

2. An Improved Locator, W. E. Gilson, OSRD 1235, 
University of Wisconsin, March, 1943. 

Div. 17-221-M2 

3. Gilson's Surgeon's Metal Locator, R. H. Maxson, 
OEMsr-1401, Burdick Corporation, May 22, 1945. 

Div. 17-221-M3 


CONFIDENTIAL 



OSRD APPOINTEES 


DIVISION 17 

Chiefs 

George R. Harrison 
Paul E. Klopsteg 


Deputy Chiefs 

Dr. E. A. Eckhardt 
Paul E. Klopsteg 


Technical Aides 

Miles J. Martin Clark Goodman 

Francis L. Yost John A. Hornbeck (WOC) 


Members 

0. S. Duffendack 
Theodore Dunham, Jr. 

E. A. Eckhardt 
Harvey Fletcher 

Melville I. Stein 


William E. Forsythe 
George R. Harrison 
Herbert E. Ives 
Brian O’Brien 


section 17.1 

Chief 

E. A. Eckhardt 


Deputy Chief 

Herbert E. Bragg 


Special Assistant to Chief 

John A. Hornbeck 


Technical Aide 

Herbert E. Bragg 


Members 

Charles B. Bazzoni Semi J. Begun 

J. M. Cork 


CONFIDENTIAL 


211 


212 


OSRD APPOINTEES 


SECTION 17.2 
Chief 

Melville I. Stein 


Technical Aide 

George E. Beggs 


Members 

Gioacchino Failla J. C. Hubbard 

C. H. Willis 


section 17.B 
Chief 

Harvey Fletcher 


Special Assistants to Chief 

William S. Gorton 
L. J. Sivian 


Acting Chief 

P. M. Morse 



Technical Aides 


William S. Gorton (WOC) 


Clifford Morgan 

* 

Members 


Davis Hallowell 


Vern 0. Knudsen 

Floyd A. Firestone 

E. C. Wente 

Stanley S. Stevens 


CONFIDENTIAL 



CONTRACT NUMBERS, CONTRACTORS, AND 
SUBJECT OF CONTRACTS 

The contract information given below is for Division 17 v^ork 
reported in (or related to) this volume. Contract information 
associated with Division 17 work reported in other volumes of 
the Division 17 Summary Technical Report is given in those 
volumes. 

The work under contracts whose numbers are marked with 
an asterisk (*) is not discussed anywhere in the Division 17 
STR. For details of such work the reader is referred to the 
NDRC Bi-Monthly Summaries. The contracts themselves are 
listed here for completeness of contract information. 


Contract 

Number 

Name and Address 
of Contractor 

Subject 

NDCrc-64* 

Princeton University 

Princeton, New Jersey 

Studies and experimental investigations in 
connection with Submarine Mines. 

NDCrc-81 

The Union Switch and Signal Company 
Swissvale, Pennsylvania 

Studies and experimental investigations in 
connection with circuits used to operate 
submarine mines at the Submarine Mine 
Depot, Fort Monroe, Virginia. 

NDCrc-99 

Gulf Research and Development Company 
Pittsburgh, Pennsylvania 

Studies and experimental investigations in 
connection with the development of a 
magnetic detector responsive to changes in 
magnetic fields and designed to indicate 
the approach of ferromagnetic objects. 

NDCrc-111 

The Johns Hopkins University 

Baltimore, Maryland 

Studies and experimental investigations in 
connection with the development of an in¬ 
strument for detecting and measuring 
small concentrations of noxious gases in 
the atmosphere with particular regard to 
building an instrument which is portable, 
simple to operate, and reliable. 

NDCrc-158* 

Drexel Institute 

Philadelphia, Pennsylvania 

To perform consultation and investigational 
work in connection with the studies of the 
behavior of submarine mines when sub¬ 
ject to tidal and wave action and the 
shock wave of an exploding neighboring 
mine. 

NDCrc-187 

Carnegie Institution of Washington 
Washington, D. C. 

Studies and experimental investigations in 
connection with (1) problems, such as 
those of magnetic compensation, and of 


optimum location, arising in conjunction 
with the use of magnetic compasses in 
tanks and other vehicles; (2) similar 
problems which may arise in conjunction 
with the use of magnetic compasses in 
Naval craft; (3) continued consultation, 
tests, and redesign of vehicular odographs, 
marine odographs, and aircraft odographs, 
pedographs, and dead reckoning tracers 
generally; (4) the development of a 


CONFIDENTIAL 


213 






CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS ( Continued) 


Contract 

Number 


NDCrc-199* 


OEMsr-47 


OEMsr-95 


OEMsr-150 


OEMsr-151 


Name and Address 
of Contractor 


Massachusetts Institute of Technology 
Cambridge, Massachusetts 


Massachusetts Institute of Technology 
Cambridge, Massachusetts 


Gulf Research and Development Company 
Pittsburgh, Pennsylvania 


Hazeltine Service Corporation 
New York, New York 


Subject 


detonating mechanism for use with explo¬ 
sive charges, which mechanism is to be 
actuated by the magnetic field of a tank 
or other vehicle; (5) the study of the 
magnetic characteristics of vehicles, or 
Naval craft, when necessary in connec¬ 
tion with (1), (2), (3) and (4) hereof; 
(6) the development of a detector for 
magnetic masses, such detector to be free 
of any substantial external field of its 
own; and (7) such other related problems 
as may arise from time to time. 

Studies and experimental investigations in 
connection with the development of suit¬ 
able apparatus for the measurement of 
stresses and strains produced in sub¬ 
merged metallic objects by underwater 
explosions. 

Studies and experimental investigations in 
connection with the development of a de¬ 
vice for arming controlled submarine 
mines by means of acoustic waves emitted 
by an approaching vessel. 

Studies and experimental investigations in 
connection with the development of an im¬ 
proved magnetic device for use of mag¬ 
netic land mines. 

To conduct . . . studies and experimental 
investigations in connection with anti-tank 
mine locators, to investigate all likely 
methods, apparatus, and circuits adaptable 
for use in locating metallic anti-tank 
mines, to investigate the application of 
any device developed in connection there¬ 
with to possible nonmetallic mines having 
a small metallic fuse box . . . and to de¬ 
liver working models of any devices de¬ 
veloped hereunder. 


Carnegie Institution of Washington Studies and experimental investigations in 

Washington, D. C. connection with (i) the magnetic field at 

various depths beneath and around tanks 
and motorized vehicles, (ii) the effects of 
deperming and degaussing on these mag¬ 
netic fields, (iii) the most suitable loca¬ 
tion for degaussing coils on motorized 
vehicles from practical and theoretical 
points of view, (iv) the development of a 
mechanism operated by the magnetic fields 
of vehicles suitable for the discharge of 
land mines to test the effectiveness of pro¬ 
tection, and (v) the development of a 
method of detecting land mines in ferro¬ 
magnetic cases, particularly such methods 
as may be employed in connection with 
the operation of motorized units. 


214 


CONFIDENTIAL 







CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS ( Continued) 


Contract Name and Address 

Number of Contractor Subject 


OEMsr-178 


OEMsr-266 


OEMsr-295 


OEMsr-328 


OEMsr-419 


The Johns Hopkins University 
Baltimore, Maryland 


Gulf Research and Development Company 
Pittsburgh, Pennsylvania 


Massachusetts Institute of Technology 
Cambridge, Massachusetts 


The Union Switch and Signal Company 
Swissvale, Pennsylvania 


Studies and experimental investigations in 
connection with (i) methods of infrared 
detection of gases and the development of 
apparatus for detecting CO, CO 2 , and Ho 
in submarines, together with additional 
equipment for detection of toxic gases, 
and the increase of the sensitivity of the 
instruments developed hereunder, (ii) the 
construction of up to five (5) of each of 
two (2) kinds of instruments, one a selec¬ 
tive gas detector, and the other a non- 
selective detector, (iii) the modification of 
the design of the infrared gas detector 
to give shorter reaction time and to pro¬ 
vide air cooling, and the construction of 
at least one (1) model thereof, and (iv) 
the design of an instrument suitable for 
the measurement of small concentrations 
of water vapor. 

Studies and experimental investigations in 
connection with the development of (i) 
improved methods of submarine mine con¬ 
trol, (ii) a security device, (iii) an im¬ 
proved helium purity indicator for use in 
range-finders and lighter-than-aircraft, 
(iv) a device for the determination of 
the quantity of fuel in the tanks of air¬ 
craft, (v) an indicator mine and asso¬ 
ciated devices and methods for determin¬ 
ing the effectiveness of various explosive 
means of clearing minefields, and (vi) 
other instruments and devices of warfare 
when and as requested in writing by the 
Contracting Officer or an authorized 
representative. 

Studies and experimental investigations in 
connection with the development of a de¬ 
tector of approaching vessels which will 
operate from the reflection of a sound 
wave which has been emitted by an 
anchored submarine mine, and which is 
received back within a time interval cor¬ 
responding to the danger range of that 
mine. 

Studies and experimental investigations 
looking toward the development of a pilot 
model of a circuit for controlling harbor 
mines based on a frequency selection 
method capable of operating two (2) sets 
of thirteen (13) mines from a single con¬ 
trol panel, and such further studies on 
control circuits for influence mines as may 
be requested by the Contracting Officer or 
his authorized representative. 


The Regents of the University of Wisconsin Studies and experimental investigations in 
Madison, Wisconsin connection with the development, in co¬ 

operation with approved electronics 
engineers, of an improved instrument for 
locating metal fragments imbedded in 
human tissue. 


CONFIDENTIAL 


215 






CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS ( Continued) 


Contract Name and Address 

Number of Contractor 

Subject 

OEMsr-667 Brown University- 

Providence, Rhode Island 

To conduct studies and experimental in¬ 
vestigations in connection with the im¬ 
provement of existing sound ranging 
systems and more particularly, to estimate 
impartially their capabilities, offer sug¬ 
gestions for their improvement, and indi¬ 
cate the direction to be taken by funda¬ 
mental development work. 

OEMsr-734 Duke University 

Durham, North Carolina 

To conduct . . . studies and experimental 
investigations in connection with the 
problem of ranging enemy guns by sound 
and, more particularly, to (i) study and 
develop gun ranging and locating systems; 
(ii) collect phonographic and oscillo¬ 
graphic records of the sounds of gun fire, 
the ballistic noise of projectile and other 
battle noises; and (iii) develop apparatus 
for gun ranging systems. 

OEMsr-749 Kannenstine Laboratories 

Houston, Texas 

Studies and experimental investigations in 
connection with the development of de¬ 
vices and methods for locating unexploded 
bombs, in cooperation with the Bomb Dis¬ 
posal School, Aberdeen Proving Ground, 
Aberdeen, Maryland. 

OEMsr-958 S. A. Scherbatskoy 

Tulsa, Oklahoma 

Conduct studies and experimental investiga¬ 
tions in connection with and construct 
models of devices to locate nonmetallic 
mines by neutron and seismic methods. 

OEMsr-998 Sun Oil Company 

Philadelphia, Pennsylvania 

Studies and experimental investigations in 
connection with the use of high frequency 
electric currents for the purpose of 
locating concealed metallic and non¬ 
metallic objects. 

OEMsr-1035 Leeds & Northrup Company 

Philadelphia, Pennsylvania 

Studies and experimental investigations in 
connection with (i) the engineering, de¬ 
sign, development, and construction of 
five (5) pilot units of a gas analyzer, (ii) 
the means for improving the design of the 
filter cones, methods of standardizing such 
units, and to cooperate in the study of 
and application to various problems for 
which this instrument is suited, (iii) the 
redesign of the gas analysis instrument 
to increase the sensitivity by including 
vacuum thermopiles and energy sources, 
and the construction of five (5) pilot 
models of the redesigned instrument, and 
(iv) the investigation of such other means 
of improvement as may be mutually 
agreed upon between the Contractor and 
the Scientific Officer. 

OEMsr-1042 The Maico Company, Incorporated 

Minneapolis, Minnesota 

Conduct (i) studies and experimental in¬ 
vestigations in connection with and pro¬ 
duce ten (10) models of the Gilson type 
of Surgeon’s Metal Locator, including 
such circuit and other redesign as may 
be agreed upon, and (ii) field trials 
thereof. 


216 


CONFIDENTIAL 











CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS ( Continued) 


Contract 

Number 

Name and Address 
of Contractor 

Subject 

OEMsr-1061 

Radio Corporation of America 

RCA Victor Division, Camden, New Jersey 

Studies and experimental investigations in 
connection with (i) certain acoustic and 
electric methods of locating land mines, 
particularly nonmetallic units, (ii) the 
construction of one or more models of any 
apparatus developed, and (iii) the con¬ 
struction of seven (7) additional units of 
the improved UHF nonmetallic mine 
detector. 

OEMsr-1063 

Electro-Mechanical Research, Incorporated 
Houston, Texas 

Studies and experimental investigations in 
connection with (i) potential drop ratio 
methods of locating land mines, (ii) alter¬ 
nating current inductive methods, par¬ 
ticularly in the frequency range of 100 to 
500 kc, and (iii) redesign for production 
the prototype model of the induced earth 
current detector developed hereunder. 

OEMsr-1076 

Marmon-Herrington Company 

Indianapolis, Indiana 

Studies and experimental investigations in 
connection with the development of 
mechanical means for the clearing of anti¬ 
tank mine fields. 

OEMsr-1114 

The Texas Company 

Houston, Texas 

Studies and experimental investigations in 
connection with the development of a de¬ 
tector for anti-tank mines by means of 
the masking effect of the mine on the 
natural radio-activity of the earth. 

OEMsr-1156 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 

Studies and experimental investigations in 
connection with possible methods of 
locating enemy mines by means of devices 
using sources of neutrons and detectors 
for the reflected or scattered neutrons in¬ 
cluding the design of such special de¬ 
tectors. 

OEMsr-1234 

Carnegie Institute of Technology 

Pittsburgh, Pennsylvania 

(i) Conduct studies and model experiments 
in connection with establishing funda¬ 
mental data required for improving the 
design of flailtype mine exploders and (ii) 
make a preliminary investigation of a 
sound pressure type of fuel quantity gage, 
particularly for aircraft. 

OEMsr-1374 

Polytechnic Institute of Brooklyn 

Brooklyn, New York 

Studies and experimental investigations in 
connection with the various aspects of the 
propagation of UHF radiation in soil, and 
the development of devices for the pur¬ 
pose of detecting land mines based on such 
investigations. 

OEMsr-1401 

The Burdick Corporation 

Milton, Wisconsin 

Construct ten units of the Gilson portable 
metal locator. 

OEMsr-1463 

Shell Oil Company, Incorporated 

Houston, Texas 

Studies and experimental investigations in 
connection with the methods of detecting 


all types of mines by means of currents 
introduced into the ground other than 
those currents induced by the detecting 
device itself. 


CONFIDENTIAL 


217 






CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS {Continued) 


Contract 

Number 

Name and Address 
of Contractor 

Subject 

OEMsr-1470 

The Elflex Company 

Houston, Texas 

Studies and experimental investigations in 
connection with methods of detecting all 
types of mines by means of currents in¬ 
troduced into the ground other than those 
currents induced by the detecting device 
itself. 

OEMsr-1489 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 

Studies and experimental investigations in 
connection with the development of means 
for locating fragments of methyl 
methacrylate or similar materials when 
embedded in the human body. 


218 


CONFIDENTIAL 







SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Executive 
Secretary, NDRC, from the War or Navy Department through 
either the War Department Liaison Officer for NDRC or the 
Office of Research and Inventions (formerly the Coordinator of 
Research and Development), Navy Department. Service Projects 
marked with an asterisk (*) are not discussed anywhere in the 
Division 17 STR. For details of such work the reader is referred 
to the NDRC Bi-Monthly Summaries. 


Service 

Project 

Number 

Subject 

AC-123 

Army Projects 

Development of instrument for detecting methyl methacry¬ 
late particles in the human body. 

CAC-1 

An influence operated mine for defense against submarines 
(later OD-69). 

CAC-4* 

CAC-5* 

Natural phenomena forces recorder. 

Explosive-wave phenomena recorder (mine explosions) 
(later OD-70). 

CAC-7 

Frequency selection system for controlled submarine mine 
(later OD-72). 

CE-4 

CE-31 

CE-31 Ext. 

Anti-tank detection device. 

Development of a method of detecting explosives. 
Development of a combined metallic and nonmetallic mine 
detector. 

CE-31 Ext. 
CE-32 

CE-32 Amend. 
CE-32 Ext. 

Portable underwater mine detector. 

Minefield clearing devices. 

Assistance of NDRC on blast research problems. 

Design of an electrical circuit for firing a projected line 
charge developed for clearing paths through mine fields. 

OD-46 

Development of methods for the protection of tanks against 
anti-tank land mines and techniques for testing these 
methods. 

OD-63 

OD-69 

Bomb disposal technical assistance. 

Influence operated controlled submarine mine (formerly 
CAC-1). 

OD-70* 

Explosive-wave phenomena recorder (mine explosions) 
(formerly CAC-5). 

OD-72 

Frequency selection system for controlled submarine mine 
(formerly CAC-7). 

OD-133 

OD-134 

SOS-13 

The study of the design of flails. 

Self-propelled mine clearing devices. 

Gun ranging. 

MC-100 

MC-100 Ext. 

Navy Projects 

Sound ranging equipment for the Marine Corps. 

Procurement of dodars; 100 acoustic coupler tubes, to adapt 
the T-21-B microphones to dodar use. 


CONFIDENTIAL 


219 













INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. For 
access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


Absorption bands, gas detectors, 
infrared, 176-177 
Acoustic coupler, 98, 109 
Air layers, sound transmission prop¬ 
erties, 158-159 

Amphibious snake (M4 demolition 
clearance device), 65 
Analyzers, gas 

see Gas detectors, infrared 
Anemometers for wind speed meas¬ 
urement, 150 

AN/PNS-1 (improved time interval 
dodar, D-3), 131-133 
design, 131 
development, 131-132 
instrument cases, 131-132 
requirements, 131-132 
test results, 131-133 
AN/PRS-1 (XB-4) land mine de¬ 
tector, 3, 20-28 
description, 20-21 
improvements, 25-27 
performance, 23 

properties of mine materials, 24 
reflection effects, 21 
reliability, 23 

vulnerability to mechanical shock, 
23 

AN/PRS-2 (Mamie) land mine de¬ 
tector, 33-34 

AN/PRS-5 (XB-2) land mine de¬ 
tector for beaches, 4, 29-31 
AN/PRS-6 (XB-6) land mine de¬ 
tector, 3-4, 38-40 
combination detector, 40 
metallic portion, 40 
military requirements, 38 
nonmetallic portion, 38-40 
Anti-boat mine detector (AN/PSS- 
1), 16-17 

Anti-countermeasures for land mine 
detection, 42-45 

Anti-infiltration listening system, 
164-166 

Anti-personnel mines, 2, 18 
Anti-tank mines, 2, 21 

duplication of enemy mines, 65-66 
dynamic tests, 48, 57-58 
M-5 nonmetallic, 21 
magnetic, 71-73 
point charges, 48, 60-62 
shock tube studies, 49, 61-64 
static tests, 57 


Artillery plotting grids for sound 
ranging data, 122-123 

Artillery ranging 
see Sound ranging 

Asymptote method, computation of 
sound ranging data, 113-114 

BAD (Brewster angle detector), 
25-27 

Ballistic-burst method of sound 
ranging 

applications, 116 

field tests, 118 

gun records, 166-168 

line of flight determination, 116 

ranging, 116-118 

summary, 98 

templates and tables, 123-125 
wave characteristics, 106, 167- 
168 

Beach mine detector, AN/PRS-5 
(XB-2), 4, 29-31 

Bell Telephone Laboratories, dodar 
research, 127 

Binaural listening devices 

anti-infiltration warning device, 
164-166 

binaural outpost (Binop), 98, 
111-112 

Bomb fragment detection in human 
tissue, 182-186 
animal experiments, 182-183 
radioactive coating, 183 
summary, 182 
x-ray, low voltage, 184-186 
x-ray opacification, 186 

Bomb location, 187-192 
electric methods (r-f), 192 
history, 191-192 
instruments, 187 
magnetic methods, 187-192 
potential drop ratio method, 192 
seismic methods, 192 
summary, 187-188 
thermal methods, 192 
vertical-horizontal gradiometer, 
187-188 

vertical-vertical gradiometer, 187, 
191-192 

Bore-hole gradiometer 
description, 188-190 
operation, 190-191 
summary, 187-188 


Brewster angle detector (BAD), 
25-27 

Bullet detectors for the human 
body, 201-202 

Burst wave (detonation wave), 106 

Calibration of land mines, 60-64 
mine tests, 62-63 
point charges, 56-61 
pressure measurements, 62 
shock tube, 61-64 
universal indicator mines, 60 
Caliper-type metal locator, 201-202 
Carbon dioxide, infrared detection 
and measurement, 181 
Carbon monoxide, infrared detec¬ 
tion and measurement, 178- 
179, 181 

CIW marine magnetometer, 67-71 
Corrugation in acoustic waves, 140, 
157-158, 163-164 

Countermeasures for land mine de¬ 
tection, 42-45 
Coupler, acoustic, 98, 109 

D-2 time interval dodar 

see Time interval dodar, D-2 
D-3 improved time interval dodar 
see Time interval dodar, im¬ 
proved, D-3 

Degaussing of vehicles, 74-75 
Demolition clearance techniques 
amphibious snake, M4; 65 
anti-tank mines, 57-58 
evaluation, 66 
indicator mines, 58-60 
point charges, 60-62 
projected line charge firing de¬ 
vice, 64-65 
shock tube, 61-64 
summary, 48-50 

Detection of anti-boat mines, 16-17 
Detection of concealed weapons, 
193-200 

Detection of gas by infrared 
method 

see Gas detectors, infrared 
Detection of plastic particles in 
human tissues, 182-186 
Detonation wave (burst wave), 106 
Dinah, nonmetallic mine detector 
program, 34 


CONFIDENTIAL 


221 


222 


INDEX 


Dodar 

coordination of sub-bases, 136 
gun location errors, 137 
impulse measurements, 147-148 
meteorological errors, 137-138 
recorder type, 127-129 
time interval, D-2; 129-131 
time interval, improved, D-3; 
131-133 

ultra lightweight, 133 
Doppler method for sound ranging, 
99, 126-127 

Drop-weight method, demolition 
clearance, 57 

Dry paper recorder, sound ranging, 
98, 110-111 

Earth current detector (ECD), 5, 
31-32 

Eccles-Jordan trigger circuit, 145 
Electric (r-f) methods of bomb 
location, 192 

Electromagnetic detecting device, 
193-200 

description, 193-198 
development, 193 
history, 198-200 
requirements, 193 
Electromechanical mine detection 
bomb location, 192 
instruments, 4-5, 28-29 
sound ranging, 99, 125-126 
Enemy hide-outs, location, 171-173 
Enemy mines, duplications, 65-66 
Enemy movements, detection, 164- 
166 

Exploder (mine), flail-type, 47-55 
design, 47 

initial studies, 51-52 
rigid extensions, 54-55 
self-rotating, 54 
summary, 47-50 
test results, 52-54 

Field records in gun ranging 
analysis, 101, 166-168 
Firing device for magnetic mines, 
75-78 

electronic, 77-78 
mechanical, 75-77 

Firing device for projected line 
charge (PLC), 64-65 
Flail-type mine exploder, 47-55 
design, 47 

initial studies, 51-52 
rigid extensions, 54-55 
self-rotating, 54 
summary, 47-50 
test results, 52-54 


Fort Bragg, North Carolina 

ballistic-burst method of sound 
ranging, 118 

seismic method for sound rang¬ 
ing, 125-126 

Frequency control system for sub¬ 
marine mines, 79-95 
history, 91-92 

magnetic detector, 82-87, 92-93 
military requirements, 80 
sonic detector, 88-91, 93-95 
summary, 79-82 

Gamma-ray detector for friendly 
mines, 32, 33 

Gas detectors, infrared, 176-181 
applications, 181 

CO detector in airplane ventila¬ 
tion, 179, 181 

C0 2 in submarine atmospheres, 
~ 181 

gas mask testing, 181 
measurement by absorption, 177- 
179 

model IV, 177-181 
model V, 179 
model VI, 179-181 
nonselective type, 176 
principles, 176-177 
selective type, 176, 177-181 

Goodwin method of sound ranging, 
119 

GR-3-C, gun ranging equipment, 
107-112 

Gradiometers, bomb-locating, 187- 
191 

vertical-horizontal (VHG), 187- 
191 

vertical-vertical (VVG), 187, 
191-192 

Gun location errors, 137 

Gun ranging 

see Sound ranging equipment; 
Sound ranging techniques 

Hedden metal locator principle 
(SCR-625), 9 

Helmholtz correction for wave 
front irregularities, 157-158 

Hydrodynamics in sound transmis¬ 
sion, 158-160, 163 

Improved time interval dodar, D-3; 
131-133 
design, 131 
development, 131-132 
instrument cases, 131-132 
requirements, 131-132 
test results, 131-133 


Impulse sources of sound, 141-142, 
147-148, 155 
Indicator mines 
calibration, 60 
evaluation, 48, 50, 66 
Ml universal indicator mine, 58- 
60 

M2 universal indicator mine, 58 
shock tube studies, 62-64 
TMi-43 indicator, 57-58 
Influence-type mines, submarine 
history, 91-92 

magnetic detector, 82-87, 92-93 
military requirements, 80 
sonic detector, 88-91, 93-95 
summary, 79-82 
Infrared gas detectors 

see Gas detectors, infrared 
Iron detectors 

see also Metallic mines, detection 
electromagnetic device, 193-200 
portable device, 4, 16-17 

Kannenstine Laboratories, bomb 
locating instrument, 187 
Kelley, J. B., gun wave frequencies, 
108 

Land mine detectors, 1-45 
anti-countermeasures, 6, 42-45 
countermeasures, 6, 42-45 
evaluation, 6-7 
for beaches, 29-31 
metallic, 7-18 
nonmetallic, 18-38 
summary, 1-2 
universal, 38-40 
Land mine detectors, types, 3-6 
AN/PRS-1; 20-28 
AN/PRS-2 (Mamie), 33-34 
AN/PRS-5 (XB-2), 29-31 
AN/PRS-6 (XB-6), 38-40 
Land mines, calibration, 60-64 
mine tests, 62-63 
point charges, 56-61 
pressure measurements, 62 
shock tube, 61-64 
universal indicator mines, 60 
Land mines, magnetic, 75-78 
Land mines, mechanical and dem¬ 
olition clearance 

see Demolition clearance tech¬ 
niques; Rotaflail 

Ml universal indicator mine, 58 
M2 universal indicator mine, 58 
M4 amphibious snake for demolition 
clearance, 65 

M4 medium tank, magnetic field, 
71-73 


CONFIDENTIAL 



INDEX 


223 


M5 nonmetallic mine, 21 

M12()/TN type microphone, 135 
M13Q/TN type microphone, 135 
Macro-meteorology in sound rang¬ 
ing, 139-140, 148-149 
Magnetic bomb location, 187-192 
Magnetic detectors for land mines 
see Land mine detectors 
Magnetic detectors for submarine 
mines 

description, 82-83 
detector tube, 82-83 
development, 80-81 
“firing” frequency, 85 
harbor-defense network, 84-85 
history, 92-93 

magnetic fields of ship, 85-87 
oscillator, 83 
shore equipment, 84-85 
test frequency, 83-84 
test sets, 93 

Magnetic fields of vehicles, 67-75 
degaussing, 74-75 
induced field, 72 
M-4 medium tank, 71-73 
measurements, 67-71, 73-74 
permanent field, 72 
Magnetic land mines, 75-78 
electronic firing device, 77-78 
mechanical firing device, 75-77 
Magnetic methods of weapon¬ 
detecting, 193-200 

Magnetometer, CIW Marine, 67-71 
Mamie (AN/PRS-2) land mine de¬ 
tector, 33-34 

Marine Corps recorder, sound rang¬ 
ing, 111 

Mast equipment for field sound 
measurements, 147 
Mesick, J., V H method for meteor¬ 
ological corrections, 191 
“Met” errors, sound ranging, 139- 
140, 148-149 

Metallic mines, detection, 2-4, 7-18 
AN/PSS-1; 16, 17 
equipment design principles, 7-8 
evaluation, 17 
military requirements, 8 
phase discrimination locators, 
13-16 

portable iron detector, 16, 17 
SCR-625; 3, 8-13 

Metallic weapons, detection, 193- 
200 

Meteorological corrections in sound 
ranging 

calibrating systems, 151 
comparison of methods, 119 
Goodwin method, 119 


micro-meteorology, 170-171 
principles, 169 
reduction of errors, 137-139 
standard correction, 119 
summary, 168 
temperature, 149 
triangular array method, 119 
two-dimensional microphone 
arrays, 170-171 
types of errors, 139-140 
V H method, 119 

wind speed and direction, 149- 
151 

Micro-meteorology, sound ranging, 
170-171 

Microphones for sound ranging 
arrays, two-dimensional, 170-171 
bases, 106-107 
description, 135 
development, 134-135 
frequency ranges, 136 
lightweight crystal, 100, 133-136 
modifications, 108-110 
requirements, 134 
Signal Corps type M-12()/TN, 
135-136 

Signal Corps type M-13()/TN, 

135 

sound ranging, 98, 107-110 
T-l dodar, 133, 134 
T-2 dodar, 135-136 
T-21-B condenser, 98, 134 
T-23 hot wire, 134 
test results, 135-136 

Mine clearing devices, mechanical 
see Demolition clearance tech¬ 
niques 

Mine detection 

land mines, 1-7, 38-45 
metallic mines, 2, 3, 7-18 
nonmetallic mines, 3, 4, 18-38 
universal, 38-40, 58-60 

Mine exploders, mechanical, 46-48, 
51-56 

advantages, 55 
limitations, 55-56 
military requirements, 46-47 
Rotaflail, 47-48, 51-55 
Springflail, 47-48, 55 

Multiple short base method, sound 
ranging, 99, 127 

Multipolar coordinate method for 
sound ranging data, 114-115 

Muzzle wave (gun wave) in sound 
ranging, 106 

Neutralizing magnetic fields of 
vehicles, 74-75 


Nomograms for sound ranging 
advantages, 122 

case of one microphone inopera¬ 
tive, 115 
equipment, 120 
evaluation, 116 

multipolar coordinate method, 
114-115 

summary, 98-99 

Nonmetallic mines, detection, 3-5, 
18-38 

AN/PRS-1; 3, 20-28 
beach detector AN/PRS-5 (XB- 
2), 29-31 

Brewster angle detector (BAD), 
25-27 

development, 18-19 
Dinah, 33, 34 

earth current detector (ECD), 
31-32 

evaluation, 35-38 
Mamie (AN/PRS-2), 33-34 
military requirements, 19 
oscillation, uhf, 20, 21, 25-27 
radioactive detectors, 5, 32-35 
seismic (electro-mechanical) de¬ 
tector, 4-5, 28-29 

Oscillogram, sound ranging equip¬ 
ment, 103-105 

Paper recorder for sound ranging, 
98, 110-111 

Personnel services, sound ranging 
project, 173 

Phase discrimination locators, 13-16 

Phase measurement systems, single 
frequency, 143-145 
Eccles-Jordan trigger circuit, 
143-145 

broken circle, 143 
Lissajous figures, 143 
“step-pattern,” 143-144 

PID (portable iron detector), 4, 
16-17 

Plastic bomb fragment detection in 
human tissue, 182-186 

PLC (projected line charge), dem¬ 
olition clearance, 64-65 

Plexiglas particles, detection in 
human tissue, 182-186 

Plotting grids for sound ranging 
data, 99, 120-123 
artillery, 122-123 
nomogram, 120-121 

Point charges, effect on anti-tank 
mines, 60-62 

Portable iron detector (PID), 4, 
16-17 


CONFIDENTIAL 




INDEX 


Potential drop ratio method of 
bomb location, 192 
Potentiometer, wind direction in¬ 
dicators, 150 

Probe-type locators, 201-202 
Projected line charge (PLC), dem¬ 
olition clearance, 64-65 

Radiation detectors 

see Gas detectors, infrared 
Radioactive detectors for friendly 
mines, 5, 32-35 

Geiger-Mueller gamma ray de¬ 
tector, 33 

Mamie (AN/PRS-2), 33-34 
Rahm Instrument Company, single 
frequency sound measure¬ 
ment recorder, 145 
Receiver unit (RU), electromag¬ 
netic detecting device, 196- 
198 

Recommendations for future re¬ 
search 

land mine detection, 44-45 
sound ranging, 173-175 
Recorders (dry paper), sound rang¬ 
ing, 110-111 

Recorder-type dodar, 127-129 
Rotaflail (mechanical mine ex¬ 
ploder), 47-55 
design, 47 

initial studies, 51-52 
rigid extensions, 54-55 
self-rotating, 54 
summary, 47-50 
test results, 52-54 

SCR-625 (metal detector), 3, 8-13 
Sea mine detector, 16-17 
Seismic detection 
bomb location, 192 
instruments, 4-5, 28-29 
sound ranging, 99, 125-126 
Self-rotating Rotaflail, 54 
Shepherd, W. H., phase measure¬ 
ment method, 143-145 
Shock tube technique for demolition 
clearance, 61-64 
calibration, 62 
description, 62 
mine tests, 62-63 

Signal Corps type M-12 and M-13 
()/TN microphone, 135-136 
Sommerfeld’s electromagnetic 
theory for sound propaga¬ 
tion along a boundary, 162- 
163 

Sonic detectors 
development, 80, 81 


echo-type, 81, 94-95 
history, 93-95 
military requirements, 80 
transducers, 94 
use, 88-91 

Sound propagation along a bound¬ 
ary, 161-163 

Sound ranging equipment, 98-100 
anemometers, 150 
artillery plotting grids, 122-123 
ballistic-burst templates and 
tables, 123-125 
binaural outpost, 111-112 
dodar, 127-133 
dry paper recorder, 110-111 
Marine Corps recorder, 111 
meteorological instruments, 148- 
151 

microphones, 107-110, 133-136, 

143, 147 

nomograms, 120-122 
oscillograms, 103, 104 
phase measuring systems, 143 
receiving amplifiers, 143 
recording equipment, 145, 147 
trace reading templates, 120 
wind speed and direction indi¬ 
cators, 150 

Sound ranging techniques, 98-100 
analytical computation, 113-114 
ballistic-burst methods, 116-118 
coordination systems, 136 
doppler effect method, 126 
impulse source measurements, 
141-142, 147-148 

meteorological corrections, 118- 
119, 137-138, 148-151 
microphone bases, 106-107 
multiple short base method, 127 
multipolar coordinate method, 
114 

nomographic computation, 114- 
116 

probability of gun location errors, 
137 

seismic method, 125-126 
single frequency measurements, 
141-147 
Sphinx, 101 

standard plotting method, 102- 
106, 112 

Sound transmission theory, 152-164 
air layer, 158-160 
boundary conditions, 161-163 
Helmholtz correction for wave 
front distortion, 157 
hydrodynamics, 159, 163 
impulse sources, 155 
intensity loss, 162-163 


interference field of two plane 
waves, 161 

‘‘met” correction, 152 
micro-met fluctuations, 153 
phase difference, 153-155 
phase-amplitude relationship, 
161, 163 

point source, 162-163 
rectilinear vortex, 163-164 
reflection coefficient, 161-162 
regional effectiveness, 164 
scattered sound, 164 
single frequency sources, 153-155 
stationary medium, 160-161, 164 
temperature gradient, 160 
velocity measurements, 163 
vertical structure effects, 155-156 
wave amplitude, 153-155 
wave front corrugations, 157-158, 
163-164 

Sperry Gyroscope Company, phase 
measurement method, 143- 
144 

Sphinx, location of enemy hideouts, 
101, 171-173 

Springflail (mechanical mine ex¬ 
ploder), 47-48, 55 

Submarine influence mines, fre¬ 
quency control system, 79-95 
history, 91-92 

magnetic detector, 82-87, 92-93 
military requirements, 80 
sonic detector, 88-91, 93-95 
summary, 79-82 

Surgeon’s metal locator, 201-202 

T-l microphone, dodar system, 
133, 134 

T-2 microphone, dodar system, 135 

T-21-B condenser microphone, 98, 
134 

T-23 hot wire microphone, 134 

Temperature effects in sound re¬ 
cording, 149 

Templates for sound ranging data 
ballistic-burst, 99, 123-125 
trace-reading, 99, 120 

Terrain factors, sound propagation, 
101, 155-157 

Thermopiles 

see Gas detectors, infrared 

Time interval dodar, D-2, 100, 129- 
131 

combat reports, 131 
design, 129 
manufacture, 130-131 
military requirements, 214 
operation, 129 
tests, 131 






225 


Time interval dodar, improved, 
D-3; 131-133 
design, 131 
development, 131-132 
instrument cases, 131-132 
requirements, 131-132 
test results, 131-133 

TMi-43 (universal indicator mine), 
57-58 

Trace reading templates for sound 
ranging data, 99, 120 

Transmitter unit (TU), electro¬ 
magnetic detecting device, 
196 

Triangular array method of sound 
ranging, 119 

Two-dimensional microphone 
arrays, 170-171 

U-H-F oscillation in nonmetallic 
detectors 

see Nonmetallic mines, detection 

Universal mine detection, 38-40 
see also Indicator mines 


Vehicles, magnetic fields of, 67-75 
degaussing, 74-75 
induced field, 72 
M4 medium tank, 71-73 
measurements, 67-71, 73-74 
permanent field, 72 

Vertical-horizontal gradiometer 
(VHG), bomb location, 187- 

191 

description, 188-190 

operation, 190-191 

summary of development, 187-188 

Vertical-vertical gradiometer 
(VVG), bomb location, 191- 

192 

V H method, meteorological correc¬ 
tions, 119 

Water mine detector (AN/PSS-1), 
16-17 

Wave amplitude, effect on sound 
transmission, 153-155 


Wave front corrugations, sound 
transmission theory, 140, 
157-158, 163-164 

Weapon-detecting device, 193-200 
description, 193-198 
development, 193 
history, 198-200 
requirements, 193 
Wind speed and direction, effect on 
sound measurements, 149- 
151 

Wind speed measurements with 
anemometers, 150 

XB-2 land mine detection 
(AN/PRS-5), 4, 29-31 
XB-4 land mine detector, 3, 20-28 
XB-6 land mine detector (AN/PRS- 
6), 3-4, 38-40 

X-ray detection of plexiglas in 
human tissue, 184-186 
X-ray investigations, surgeon’s 
metal locator, 201, 202 
X-ray opacification of plastics, 186 



UHCIASS1FIED 








































































































